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.