You've never tried to get served in a regular priced UK restaurant have you?Then again there are a lot of countries that don' t have it at all, and gee, somehow they still have waiter/waitresses as good as the rest.
Sure in the high end restaurants they pay staff enough you get great service, but in the lower end joints--good luck getting served. The staff don't make any more or any less whether or not you get your food and it shows.
That said wait staff are sales people they SHOULD be paid on commission, it's their job to sell you more food and more drinks and make sure that you come back again in the future. Their income should be tied to the size of the bill, and tipping does that. Just most sales people are paid with a kickback from management rather than a tip from the customer. If you think about it in those terms tipping is a little less offensive, but for bill simplicity I would prefer they added it on the bill automatically.