It's not that the UK is different, Blue. It is that the USA is different. I understand why, tips are included in the wage scale of many underpaid workers. this is crazy, pay them what they are worth Then we pay whatever the menu says the price is. easy peasy. You are shocked when foreigners don't understand that waitresses expect a 20% tip.
When we go to a restaurant, any tip we give is voluntary. If the waiter was unpleasant , they get nothing. If they were awesome, they get 20%. IF they deserved it.