"the average interest rate paid on UK national debt has been falling steadily.

So too has the cost of the interest on that debt be(e)n falling as a part of GDP and of total government spending. Just 4p in every pound the government now spends goes on interest payments. And two-thirds of that goes to people in the UK.

In that case the idea that interest payments on the national debt will be a massive burden makes no sense at all."
https://www.taxresearch.org.uk