Labour won't get rid of Brown until after he's lost in 2010 (unless he calls the election earlier or falls under a bus or something). To do so would look both feeble and highlight the undemocratic nature of Brown's leadership. What's that? Sure, they've allready got that image, but that's just politics.
Much as it pains me to say it, Dave does seem likely (through absolutely no result of his own merit or skill) to settle comfortably into premiership at the next general election. The Tories do however have a few good people onside (compare Labour who have a well-meaning leader but numerous incompetents at the top) so there is that silver lining.
You might imagine a '92 scenario though, with the opposition leader being so dreadful that everyone still opts for the incumbent. But it won't happen.
For London, if Boris wins it the Tories will give him someone who'll do all the work.
Quinlad, I think your point is somewhat odd. If Boris does a rubbish job, it'll certainly be bad for the tories (and even then it won't damage them significantly), but it'll by no means be damaging for them to have a win in London. Politicians are good at finding scapegoats, I wouldn't worry about the tories running out any time soon. Or they'll just intensify their fire on Brown.