I mean where you draw the line is up to you, but if there are three of you x £15, plus drinks lets say total of £64 , I would normally round it up to 70. By good restaurants I meant anything that isn't fast food (Subway, McDs). But perhaps I am not a typical customer, I don't know.
Often I go out to a restaurant with a friend and order fairly common food - totalling <£30 including drinks, and not making many demands of the staff. I don't think tipping someone to do the bare bones of their job is reasonable.
On the other hand, if I were to go with a reasonably sized group of people, had to get a table big enough for the lot of us, have people with dietary requirements or order cocktails or who otherwise make many demands of the staff... it's worth tipping as we're a pain in the ass, basically. And many restaurants automatically add a service charge in that case anyway.
