English pub food is paid for at the bar with your drinks, that you yourself walk away carrying. They often give you a number on a wooden spoon and if you hate the food, you just don't go there again.
If?
With love, Frenchie
disclaimer: not French, but having spent time in both countries it's clear that tipping is nowhere near as prevalent as in the States where 20% is pretty common.
For more on Pub culture see the fascinating "Passport to the Pub: A guide to British pub etiquette" - http://www.sirc.org/publik/pub.html
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.
It could be an artefact of different sorts of relationship people feel they have with bar staff (and serving staff in general).
I think it's fairer to say that traditionally you can tip the staff by buying one for them, but it's not something you observe happening all that often.
At Christmas, maybe?
LOL, I know, UK bad food etc etc...
Actually some of the best food in the UK is to be had in pubs IMHO. Also some of the worst, if you just pick one at random you might get either end of the spectrum.
In my experience it's more normal that you order your food at the bar, and a member of staff then brings your food to your table. A tip isn't normally expected in this case.
In a gastropub your order would normally be taken at your table and you'd tip just as if you were in a restaurant.