Always. I've never had such shitty service that I wouldn't tip. I'm not one of those stuck up c*nts who needs to be pampered when eating out. If I'm dining at a restaurant I don't need anything to be above and beyond. If the service is fine and attentive then that is good enough for me. I hate it when people pest the waiters, especially if a place is busy. They are rushed off their feet, it is busy, they have many tables to serve and take orders from, give them a fucking minute. You'll get your food and another beer, you wont starve to death. You're living in a first world country and you are causing a fuss because you have to wait a bit longer for food?
When it comes to deliveries from pizza places and stuff I'll always give a couple of quid. Sometimes you'll wait ages on busy days but that likely isn't the delivery guys fault, so it wouldn't be fair not to tip on that score. In bars/pubs I'll always tell them to take their own, or if it comes to 9 quid or say 18 quid for the round I'll tell them to keep the change.
It's just the way I've been brought up and I think it's right. They are working hard, probably on low wages, and tips help. Those of you who don't tip belong in the mingebag thread