Tipping is a thing in the UK, I tip in restaurants, as long as I (who run bars and have run very-food led places) approve of the service, barstaff I'll usually buy drinks if I want to rather than tip but it's less common especially if I order at the bar (so less 'service'), and whether they pocket the money or drink it isn't my business.
Tipping in and of itself is fine, the issue is a) if they don't get the tips they don't get paid right ie, no rent, no food which seems to be the American way, and...
b) tipping as a non-customer, which I learnt about reading this thread today and seems literally insane to me! My bar had an electrician in today to service our glasswasher, the logic I see in here about musicians tipping barstaff is that he should have tipped my staff who were on so they recommend I hire him, over another sparky, next time I need a job done. That's just mad to me! Haha