What's more, the British seem to have no qualms about simply ending a series when they think there's nothing more to add.
Consider Spaced: the cast and the audience loved it, but the story had come to a conclusion after the second series and the circumstances that originally gave birth to the ideas had changed, so they refused to consider another series.
Contrast this with American shows which typically only end if they get cancelled and where success is measured in how often you can get a new series funded.
There are plenty of American shows that should have been cancelled after a single season, but weren't because they were popular.
Stuff like Prison Break, Heroes. In fact Prison Break's season 1 had a very clear, obvious conclusion (they broke out) - so there was no clear path for the story to take after that. In Britain it would not have been picked up for a second series.
Consider Spaced: the cast and the audience loved it, but the story had come to a conclusion after the second series and the circumstances that originally gave birth to the ideas had changed, so they refused to consider another series.
Contrast this with American shows which typically only end if they get cancelled and where success is measured in how often you can get a new series funded.