I think if Andy wins he should make it a double celebration and publicly propose to Kim during his victory speech. They've been together (mostly) for a few years now and they're both old enough and mature enough to know that they're ready. Kim, of course, will accept and Andy will win the hearts and minds of the entire British public (more for the proposal than the Slam). We're all romantics at heart!
Of course, he can still propose if he loses but maybe a quiet cosy restaurant would suffice.
Its strange you should say that, but I thought to myself when he wins tomorrow ! he would propose to Kim, but I think in private. Just came into my head goodness knows why?