Thing is, ITK, the Labour Party is much more nervous about ditching an unpopular leader than the Tory Party is. This is even more the case when the Party is in government.
Hard to believe, but Harold Wilson was considerably less popular in the late 60s than Blair is now. It would take a major scandal for the parliamentary party to unseat Blair; so, unfortunately, it looks like he'll be able to choose the time of his own departure.
I would, of course, love to be proved wrong!