IIRC Brown's approval ratings were unexpectedly good up until his announcement at the start of October that their wouldn't be an early election despite all the speculation that their would be that his government had fuelled. If he'd gone to the country in the autumn of 2007 his approval ratings wouldn't have dived so precipitously. It may be that friction within the party combined with fallout from the financial crisis might have seen him replaced, but I think that even if it isn't Brown going in to a post-Olympics election it would still be a winnable one if the economic recovery seemed fairly set.Unless Brown suddenly has a complete personality change any government of his is going to be simply too big a target for the opposition over the long-term, and too much of a cause of OTL-style friction within his own party. By 2012 the narratives would be already set, and the Conservatives would be starting from a slightly higher base than 2010 if we're assuming a John Major-style majority in 2007.