"Nigel"

It has to be. You won't find anyone in North America who would name a boy Nigel even if they were paid well to do so, but it's still a common name in the UK. It's classically British, in my opinion.

Another of that sort is "Lester", but I don't think it rates quite as high as Nigel.

Can anyone come up with any other given names, male or female, that come to mind which are pretty much limited to people born in the UK?