After reading some of these posts, I feel a right tight-wad! I tip my hairdresser £2. (Some here tip £10!) I always feel very embarrassed when tipping people - cab-drivers, etc. My husband is a generous 'tipper' (and I'm always telling him off!) But even he baulked at a restaurant tip, added to our bill recently.
(And why do restaurants think they have the right to just add a 'service charge' to your bill??? Surely their prices reflect the food and their services. Otherwise, we could all go into a restaurant and say, "It's okay, I'll just come into the kitchen, cook my own food and serve myself, thank you." Grrrr! makes me so mad!) 
Anyway ... back to our recent restaurant bill. They had added £30 for 'kitchen staff'! (Do they not pay them, then ...?) Husband demanded that they remove this from the bill - Waiter: "I'll have to speak to the Manager." Husband: "You'd better!" He came back with a revised bill, and we left without leaving a tip at all.
I'm sure, that if you've had a couple of glasses of wine, they think that you won't check your bill. I dread to think of how many people have been caught out by this flagrant dishonesty.