The reality (in the case of the UK) is that the answer would be we can't afford to pay, we can't be compelled to pay, and we're not going to pay. Personally I think that's the almost inevitable outcome in the case of historical legacies of this type, and I find the moral argument in favour of paying up somewhat dubious anyway.
We're never going to agree on this issue, which is fine of course.
I cant speak for all of the UK's former colonies but i expect most are just happy to have you out of their hair and happy enough to continue on their own way, relatively free of your influence. African Americans dont have that. A UK reparation bill would be hilarious though, i'd probably take you centuries to pay off.