Debt interest is now a third spending department

Every Budget since the bond-market spasm of 2022 has been written, in part, for the gilt market. The autumn event now marked for late October is no different. The United Kingdom is not a fiscal outlier on simple debt-to-GDP charts. It is an outlier in the price it pays to roll the debt it already has.

Debt interest was about £110 billion in 2025–26. That is no longer a residual. It is the third-largest item of public spending, behind health and welfare, and it is sensitive to every tenth of a percentage point on the curve. The Office for Budget Responsibility’s July fiscal-risks report put the point bluntly: UK 10-year yields have been among the highest in the G7, and the differential versus the advanced-economy average has widened.

The government’s own rules ask for public sector net financial liabilities to be falling as a share of GDP by the end of the parliament. At the end of July those liabilities were 83.7 per cent of GDP, a touch higher than a year earlier. Public sector net debt, the cruder and more familiar stock, is still near 95 per cent and, on the March forecast, was expected to drift a little higher before any later decline. Meeting a rule by a tenth of a point is not the same as having a comfortable path.

Issuance is the daily reality. The financing requirement for 2026–27 is smaller than the previous year’s swollen programme, which is the one piece of good news the Debt Management Office can offer. It remains large in absolute terms. Quantitative tightening means the Bank is no longer the reliable bid it was in the 2010s. Real money — pension funds, insurers, overseas accounts — has to want the paper.

That is why “headroom” is such a fragile idea. A few days of higher oil prices, a softer growth print, or a political argument that looks like it might become spending, and the cushion implied by the last OBR forecast shrinks. Treat every such figure as a function of the yield curve, not a pile of cash in a Treasury drawer.

There is a second, slower problem. The OBR’s long-term work now assumes that the growth-corrected interest rate, r − g, settles slightly positive. If the effective yield on the debt stock sits above the growth rate of the economy, primary surpluses become the way you stabilise the ratio. Primary surpluses are another name for tax rises, spending restraint, or both. They are also harder to deliver when debt interest is already eating the budget.

From this desk the honest sentence is this. Britain can finance itself. It cannot finance itself cheaply, and it cannot pretend that a rule which is met on a forecast is the same as a market that is relaxed. The Budget will be judged less on the adjectives in the statement than on the 10-year and 30-year gilts the following morning.