Have to disagree with your description of Brown as charismatic, Hedge. The man is dour, charmless and lacks the ability to interact with the public, unlike Blair and Cameron.
It's not even a 100% certainty that he will lead the party once Blair goes. It's been said that Dr John Reid is likely to stand against him, and his reputation as a tough home secretary is likely to appeal to the British public at the moment. Much more so, than a chancellor with a history of plundering pensions and stealth taxes.