In addition to the other comments, regarding the debt repayments to the US: The only reason it took so long was that the interest rate was fixed at such a low rate that there was no compelling reason to pay it back early - most years the interest rate was below inflation. It'd have been trivial for the UK to pay back faster, but it'd have been silly to do so all the time the debt has basically gotten cheaper and cheaper by the year.
It may have been crushing early on psychologically. Debt to GDP reached around 200% in 1945. But despite that, the UK interest payments as a percentage of GDP were far lower in 1945 (at around 6%) than after WWI, when it reached a peak of around 9%. It also rapidly dropped.
By 1956 it was down to about 4%. When the last parts of the load was being paid back it was around 2%.
And note that this was total interest on the total British national debt, not just the wartime loans.
Consider that when the debt was taken out, it represented about 10x as much in 2016 dollars as what was paid back in absolute terms including interest. The total amount paid back in 2016 dollars would be higher, as some was obviously paid back all the way back in 1950, but on the other hand, by the time of the final repayments, inflation had reduced the overall value of the repayments drastically. Overall, I believe the UK profited immensely on the loan over time - certainly for most of the period of the loan, the UK would have been able to repay in full but didn't because it received more interest on its dollar reserves than it cost to service this loan.
There are still bonds around predating the Napoleonic wars for the same reason: The interest rates are so low that you can get a better return investing the money elsewhere than by cutting your interest payments by paying off the debts.
