we always tip at a restaurant, I would stick to 10-20 % but my husband (the brit) spent years working in pubs and so he tends to tip quite well, too well if you ask me... He once left a £15 tip on a £30 meal cause the waiter was expecting a new baby. he didn't understand my point when I said so are we...
he also tends to tip taxi drivers by rounding up to the nearest note, I will round to the nearest pound, mostly because I can't be bothered to get change. although if they make me listen to a blaring radio or talk on their phones while I'm in the car I will count the pennies
My friend (british), never tips for anything. She says that she doesn't get tipped in her job, why should they...
so I'd say it depends. It isn't expected, and I use the same criteria as in the states. Couple of quid for a hair cut, 20 ish percent of the bill for a meal with good service.