I think some things are definitely cheaper in the UK - wine and beer, mostly. This is in stores. In a bar, beer is definitely cheaper, and in some places spirits & wine are, too. Restaurant wine is similarly priced, but it's hard to compare a place like Wetherspoon's to an American bar, because almost EVERYthing is dirt-cheap in 'Spoons.
I would say drinks local to where I live - Texas (only at the moment) - ARE cheaper local (obviously) than in the UK, but that's going to be true of any consumable in any location.
In America, barmen gives free drinks to customers, in the UK it's customers that give free drinks to the barmen! I think the second is in lieu of tips. If the customer feels the barman deserves a 'tip' as such, buying them a drink is a way of doing it. (and I don't know about other places, but if we got bought several drinks, we might actually only have one and pocket the money for the rest...slightly underhanded, but oh well!)