Bars in the US routinely ID people who are obviously over the age of 21. And then they get weird about foreign passports because...well, because America, I guess. I've sometimes been refused entry despite being clearly over 21 and having my passport with me (or, absurdly, been asked to show another ID, as if a passport wasn't sufficient for buying a beer). Attitudes to IDing people for alcohol are much more pragmatic in the UK.

Generally if you look over 25 in the uk you don’t need ID to prove you are over 18.