I'd say for the UK it varies - 10% or just under seems "convention", but never if service is bad, and you won't be resented if you don't as some people never tip.

In principle in the UK I'd say you don't talk about the tip - you just leave it quietly if you are going to, and it is only acknowledged in passing. I have complained about waiters who openly ask for a tip, stating clearly that asking for a tip is a guaranteed way not to get one.

Neil