Exactly. Why rush an election when he's in charge of the other side? All you are doing is giving the Labour Party a plausible reason to ditch him (crushing election defeat) and potentially start recovering sooner. If you wait out the full term, you let him do 4 more years worth of damage, and then suffer a crushing defeat, delaying the rebuild. Unless something intervenes like a new great recession and Jez leads his socialist revolution all the way to number 10. That's the gamble I guess.