jackdaw - //Why should a person resign just because he lost an election? If I stood for the Tories in Hackney against Diane Abbott I would expect to lose. Doesn't make me useless, though. //
He is't 'a person' though, he is the leader of a major political party, and if his party is defeated, then by default, he is defeated, and deemed not to be the electorate's choice.
In that position, the honourable thing to do is to resign, Foot, Kinnock, Milliband and Cameron all did it - because it is the honourabl thing to do.