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.

Really? I've seen it in heaps of places in London - adding 12.5% "gratuity" is common at many places where you'd pay ~£15+ pp.