hmm, I've never done/seen/heard of that and I've lived in London my entire life. Is it common in other parts of England? Or am I just a stingy bastard?
Even in America, part of polite bar culture is buying an especially good bartender a shot or a drink. It also makes you a quick friend who might conveniently forget a couple drinks on your bill :)
It seems common up here (York) though the reaction varies. Some decline, some places drop the value in the top jar, some may even make a point of chatting to you to drink the drink if the are not busy (though that is fairly rare).
I expect it varies as much in London, but culture tends to follow people who then follow the culture they are used to - so you may just frequent places where not tipping that way is the norm by virtue of your preferences and circle of friends.
Can't vouch for down South, but at least in the North-West it's common to say 'And your own' at the end of your drinks order. They usually only take 10-20p rather than the cost of an actual drink, though.
I think it's more common in the North. I'm a Southerner and lived in London for about 9 years and can count the number of times I've seen this on one hand, and then it's mostly been folks from the North that do it.
It could be an artefact of different sorts of relationship people feel they have with bar staff (and serving staff in general).