The UK Budget: Why £23.6 Billion of Fiscal Headroom Matters for Sterling

With the Autumn Budget approaching on 28 October, rising borrowing costs, persistent inflation and pressure on public spending have left the Chancellor with some difficult decisions. But does that necessarily mean more weakness for the Pound?

With the UK Budget now just a few days away, attention is increasingly turning towards the state of the country’s public finances and how much room Chancellor John Healey actually has to work with. This will be the first Budget under Prime Minister Andy Burnham, and while the Government may have its own priorities around taxation, spending and economic growth, the financial constraints it faces are hardly new.

The UK has struggled for years with relatively weak productivity, rising demands on public services and an ageing population. More recently, higher interest rates, the energy crisis and the conflict involving the United States and Iran have made the situation considerably more difficult. The Government now finds itself trying to support households and businesses while also convincing financial markets that borrowing remains under control.

Back in March, the Office for Budget Responsibility estimated that the Government had £23.6 billion of fiscal headroom. In simple terms, this was the amount by which the Government was expected to meet its target of covering day-to-day spending through tax revenues by 2029/30. It wasn’t £23.6 billion sitting in an account waiting to be spent, but rather a margin for error within the Government’s financial forecasts.

That distinction is important because much of that headroom is now expected to have disappeared. Higher borrowing costs and spending pressures have made the outlook less favourable, although stronger tax revenues have provided some offset. Recent estimates suggest the remaining headroom could be somewhere around £10 billion to £12 billion, but we won’t know the official position until the OBR publishes its updated forecasts alongside the Budget.

I think there are five areas worth looking at before Wednesday’s announcement, particularly when considering how the Budget could affect Sterling.

1. Government borrowing is rising, but tax revenues are holding up

One of the more concerning developments over recent months has been the amount the Government is borrowing compared with what was originally forecast.

August borrowing came in at £18.3 billion, around £3.5 billion higher than the OBR had expected. September then saw another £24.5 billion borrowed, again exceeding forecasts. Across the first six months of the financial year, borrowing reached approximately £97.8 billion, around £6 billion above the OBR’s March projections.

Those figures are clearly not ideal, but I think it is important to understand what is actually causing the deterioration.

The problem has not necessarily been that the Government is collecting less tax than expected. In fact, income tax, National Insurance and corporation tax receipts have generally held up reasonably well. Higher wages and prices have helped support revenues, even as the wider economy has shown signs of slowing.

The bigger issue has been spending, particularly debt interest, welfare payments and other government commitments. In other words, the Government is still collecting money, but its costs are rising faster.

That makes the situation slightly different from one where borrowing is increasing because the economy is contracting and tax revenues are collapsing. However, it doesn’t make the problem any less important. The Chancellor still needs to explain how these spending pressures will be managed without allowing borrowing to move further away from the Government’s targets.

2. Higher interest rates are making government debt more expensive

For me, this is probably one of the most important factors heading into the Budget.

Since March, government borrowing costs have risen considerably, partly because of the renewed inflationary pressures following the escalation of the conflict in the Middle East. UK gilt yields have moved higher, meaning the Government now faces greater costs when issuing new debt or refinancing existing borrowing.

The original analysis suggests that ten-year gilt yields were around 0.8 percentage points higher during the likely September forecasting period than they were when the March forecasts were prepared. Expectations for shorter-term interest rates have also increased, reflecting the possibility that the Bank of England may need to keep rates elevated for longer.

These changes matter because the Government has an enormous amount of debt to finance. Even relatively small increases in borrowing costs can translate into billions of pounds of additional expenditure over several years.

Some estimates suggest that the change in interest-rate expectations alone could add around £14 billion to borrowing by 2029/30 compared with the March forecast. That would consume a substantial proportion of the £23.6 billion of headroom originally available, before considering changes elsewhere in the economy.

It is also worth remembering that this has not been an exclusively British problem. Government bond yields have risen across several major economies, largely because of higher inflation expectations and global interest-rate developments.

That distinction matters when assessing Sterling. Rising UK gilt yields do not automatically mean investors have lost confidence in the Government, particularly when similar movements are happening internationally. However, the consequences for the public finances are much the same regardless of the cause.

Higher borrowing costs mean less money available for everything else, and that leaves the Chancellor with some difficult choices.

3. Inflation is a problem, but it could also help tax revenues

Inflation is another area where I think the picture is more complicated than it initially appears.

The rise in energy prices this year has significantly changed the inflation outlook. Earlier forecasts had expected inflation to gradually return towards the Bank of England’s 2% target, but the renewed pressure from energy has made that considerably less likely in the near term.

The Bank of England has already warned that inflation could rise above 4% in 2027 under a more adverse scenario, particularly if energy prices remain elevated. That creates obvious problems for households and businesses, especially when combined with the prospect of interest rates remaining higher for longer.

For the Government, however, inflation has both positive and negative effects on the public finances.

Higher inflation increases spending in areas such as inflation-linked government debt and benefits. It can also create pressure for additional support with household energy bills, particularly if the Ofgem price cap rises towards £2,000 a year from January.

At the same time, higher wages and prices can increase tax revenues. If nominal earnings rise, income tax and National Insurance receipts generally increase, while higher prices can support VAT revenues.

There is also the issue of frozen income tax thresholds. As wages rise, more people can find themselves paying higher amounts of tax or moving into higher tax bands, even if their purchasing power has not improved significantly.

The difficulty is that higher inflation can also damage the economy. Rising energy bills reduce the amount households have available to spend elsewhere, while businesses face higher costs that may affect profitability, investment and employment.

I therefore wouldn’t automatically assume that higher inflation means a significant deterioration in the Government’s fiscal position. Some of the additional spending will be offset by stronger revenues, although the overall effect will depend on how inflation feeds through into wages, consumption and economic growth.

The possibility of further energy support is also worth watching. The Government will understandably face pressure to help households struggling with higher bills, but any additional spending will need to be funded. I suspect targeted assistance is more likely than another large universal support package, particularly given how little fiscal room the Chancellor currently has.

4. The UK’s longer-term growth outlook remains a concern

Beyond the immediate issues around borrowing and inflation, the UK still has a more fundamental problem with economic growth.

One of the reasons the public finances have been under pressure for so long is that the economy has struggled to generate sustained improvements in productivity. If businesses are producing more, wages are rising and employment is growing, the Government generally collects more tax without necessarily having to increase tax rates.

Unfortunately, the UK’s productivity performance has been relatively disappointing for much of the period since the financial crisis.

The OBR reduced its medium-term productivity growth assumption from 1.3% to 1.0% in November 2025, a change which added approximately £16 billion to projected borrowing by 2029/30. That demonstrates just how sensitive the public finances are to relatively small changes in economic growth assumptions.

There is also the issue of migration and the size of the workforce.

Long-term net migration is now projected to be around 230,000 annually, compared with previous estimates closer to 340,000. Lower migration can mean slower growth in the working-age population, which in turn reduces the potential size of the economy and the tax revenues it can generate.

Of course, lower population growth can also reduce some spending pressures, so the fiscal consequences are not entirely one-sided. However, a smaller workforce generally means the economy has less capacity to grow unless productivity improves enough to compensate.

I don’t expect the Budget to provide a complete answer to the UK’s productivity problem. These are structural issues that take years to address, and the Government’s longer-term economic plans will probably tell us more about its intentions than Wednesday’s announcement.

Nevertheless, the OBR’s assumptions around growth and labour supply will be important because even modest changes can have a significant effect on the amount of fiscal headroom available.

5. How much headroom will the Chancellor choose to rebuild?

This is probably the biggest question for financial markets on Wednesday.

We know that the Government had £23.6 billion of fiscal headroom in March, and we know that the economic backdrop has become more difficult since then. What we don’t know is how much of that headroom the OBR will ultimately conclude has been lost, or how much the Chancellor intends to restore.

There is no requirement for the Government to maintain exactly £23.6 billion of headroom. The fiscal rules simply require the relevant targets to be met, and the Chancellor could technically choose to operate with a much smaller margin.

The problem is that doing so would leave the public finances more vulnerable to future economic shocks.

If the Chancellor leaves only a few billion pounds of headroom, another increase in borrowing costs or deterioration in growth forecasts could quickly put the Government back in a position where it needs to raise taxes or reduce spending.

That is why I think the amount of headroom preserved will matter just as much as the individual tax and spending announcements.

Recent economist surveys have suggested that rebuilding the buffer towards £15 billion to £16 billion could help reassure bond investors, although doing so may require additional tax revenue or spending restraint.

There is also a political calculation involved. This is the first Budget under the new Prime Minister, and the Government will understandably want to demonstrate that it is taking action on the cost of living and supporting economic growth. However, promising too much without a convincing funding plan would risk undermining the confidence it needs from financial markets.

I suspect the Chancellor will ultimately take a relatively cautious approach, with targeted measures designed to provide some support without fundamentally changing the fiscal position. Whether that is enough to satisfy households and businesses is another question, but from a currency-market perspective, credibility is likely to be more important than the size of any individual announcement.

What does this mean for Sterling?

As I have mentioned in recent Currency Desk articles, my view has been that Sterling could come under some pressure in the build-up to the Budget, but that the announcement itself may prove less disruptive than markets currently fear.

I still think that is a reasonable base case.

There has been considerable speculation over the past few weeks around tax changes, energy support, government borrowing and the amount of fiscal headroom available. Markets have had to price these risks without knowing exactly what the Chancellor intends to announce, and that uncertainty can weigh on Sterling even before any actual policy changes are made.

However, I don’t think we should automatically assume that a difficult fiscal position means the Pound has to weaken significantly after the Budget.

If the Chancellor demonstrates that spending commitments are properly funded, the fiscal rules remain achievable and there is a reasonable buffer against future economic shocks, I think there is scope for some of the uncertainty currently surrounding Sterling to disappear.

The gilt market will be particularly important here. If investors are comfortable with the Government’s borrowing plans and yields remain relatively stable following the announcement, that would be encouraging for the Pound.

Conversely, if the Budget includes significant spending commitments without convincing funding measures, or the OBR’s forecasts reveal a much weaker fiscal position than expected, the reaction could be considerably more negative.

There is also a difference between higher gilt yields caused by expectations of tighter Bank of England policy and higher yields caused by concerns about the Government’s ability to manage its finances. The former can sometimes support Sterling through higher interest-rate expectations, while the latter can undermine confidence in the currency.

For anyone with a significant Sterling requirement, I think that distinction will be particularly important over the coming days.

My expectation remains that the Government will be conscious of the risks involved in unsettling financial markets and will therefore avoid anything too dramatic. If that proves correct, we could see Sterling recover some ground once the Budget is out of the way and attention returns to the underlying economic data and the Bank of England’s interest-rate outlook.

Ultimately, Wednesday’s Budget is unlikely to resolve all of the UK’s economic challenges. Weak productivity, rising spending demands and high borrowing costs will remain issues regardless of what the Chancellor announces.

But it should give us a much clearer idea of how the Government intends to manage those challenges, how much financial flexibility it has left and, perhaps most importantly for currency markets, whether investors believe its plans are credible.

That is what I will be paying closest attention to on Wednesday, rather than simply whether an individual tax is raised or a particular spending measure is announced.