Sort of yes and mostly no.Is the UK still paying off the debts from the South Sea bubble? I recall hearing that somewhere but I'd rather hear it from an actual UK citizen who isn't a Brexiteer.
For instance the USA lent a load of money to the UK during and after WW2, which the UK had specific conditions to pay back over ~60 years. So those specific "contracts" were technically all paid off over ten years ago. However, the UK had to borrow money to meet all its financial obligations over those 60 years (including the war debt repayments to the USA), meaning that in effect that some of the debt was refinanced with new lenders rather than paid off, so it could be argued that it's still on the books.
Due to inflation and economic growth, those annual war repayments and interest ended up very, very small - under £50 million a year. To illustrate this another way, Britain runs an annual deficit of ~£50 billion per year which is larger (in nominal pounds) than its entire national debt at the end of WW2 (~£20 billion).
Thus the same argument could be made about the South Sea Bubble debt still existing (but also likewise that due to inflation and economic growth the amount owed would be so vanishingly small as to not worth thinking about). All it more realistically means is that the UK has never entirely paid off its national debt since the South Sea Bubble.