Pay As You Go makes more sense for a short visit, rather than any kind of account. I've no idea how much time is on the cards, I just top mine up at the checkout in the supermarket any time it's running low - put in five or ten quid and it lasts ages. It all depends how you use phones - my conversations rarely last more than a few seconds - "Where are you?" "Where ought I to be?"

It doesn't seem to make much difference which company you use, in my experience. O2 is as good as any.