Hung parliament. Technically, no one 'won' our election, we just had a party which was the closest to winning. Cameron's only in government because of Nick Clegg's LibDems. Constitutionally, Brown could still be Prime Minister if all the factors had worked out perfectly.
Okay, I thought that getting both the plurality and the coalition deal would make Cameron a winner rather than a "winner," but I'm not as familiar as you with the terminology in Britain. Are people really reluctant over there to call Cameron a winner?