Do US waiters really get paid more than UK with PPP/forex? You seem to have the data to hand.
That "service charge" is usually (mandated?) listed on the menu, so it would lead to quite an awkward conversation if you then complained after sitting and eating the meal. It's quite often specified for groups over a certain size and it makes more sense than getting into a "tipping" argument after the meal.
Min wage for tipped workers is $17.25 per hour. London is ~$18. So with exorbitant tipping it should be considerably higher?
Also taxes might be lower.