I live in the UK, and I personally have never given a tip, I don't think I've even seen it as an option other than physically giving the other person some cash, but AFAIK its generally seen as something you do if the service was excellent, but as I say I've never done it myself. I'm not generally in situations where it could be warranted, like I don't really eat out much or anything like that
You've never had excellent service for something?
As someone who also lives in the UK this surprises me. Almost every restaurant I eat at now takes the liberty of adding a 10% service charge onto every bill. I assume the idea of which is to make it awkward/embarrassing for you to request to have it taken off.