Well, in restaurants now the tip's already on the bill, unless you want to make your entire group or date uncomfortable by "talking to the waiter about taking it off".
Really? I rarely have dinner over 20 pounds a person (which isn't cheap, I get it, but for London prices it's also not expensive), and I haven't seen a bill without service charge added in months (excluding Whetherspoons). As soon as I don't manage to order at the register, and instead someone brings a menu, there will be a service charge.
There was one restaurant recently that didn't have one, and we almost freaked out about it.