Today federal, state, and local expenditures as a share of GDP are back at the highs reached during World War II. This time, however, we are unlikely to see a swift decrease. Wartime expenditures on items like weaponry and salaries for conscripted soldiers were relatively easy to wind down. The bulk of current and future government spending is on entitlement programs like Social Security and Medicare. This variety of spending is nearly impossible to reduce in the near term.
The relevant chart for the UK is this one from How Should Britain’s Government Spending and Tax Burdens be Measured? by David B. Smith: