I doubt any amount of charisma or good speech-making could've saved Brown - as his economic legacy came crashing down on him (there's a reason why incumbents have been dropping like flies).
It probably could've at least evened out the tide. From what I remember in 2010, Brown was incredibly lacking in charisma and looked like he was being run out of office no matter what. A little bit of strength and energy in Labour at the time may have lead to us having a Lab-Lib coalition gov right now. (Now that would be a funny scenario to watch)