Interest Payments. The cost of paying interest on the government’s debt is very high. In 2008 Debt interest payments will be £31 billion a year (est 2.5% of GDP). In 2009, they will be £35 billion (similar to defence budget). Public sector debt interest payments could be be the 4th highest department after social security, health and education. Debt interest payments are rising close to £70bn given rise in national debt.