Easy to say, but if you applied that rule in London, you'd never eat out again.

Over the past ~5 years, virtually every place I've seen which charges more than £10 for a main disk also adds a 10-12.5% "optional" service charge onto the bill, which you don't technically have to pay but have to request be removed if you wish.

The HUGE stink is that some restaurant chains (Pizza Express, for instance) actually keep a portion of this optional tip for themselves - they add it to their own bottom line!