Well, first of all, I wouldn't drop maths unless you REALLY have to. You'd be better off, I think doing maths this year and failing, then being careful about where you apply next year, possibly picking up another AS level next year?
Sorry, but you really can't afford to be picky about where you apply. You need to look at entry requirements, standard offers, what people ACTUALLY get and going for those places which you are about the same. I made that mistake of picking places I liked, without really considering that I didn't have a chance. Even if you rally love the London universities, if you think it would be high risk, you're best off only putting down one of those, and going safe for the others. At the end of the day, you will have a medical dgree, no matter where its from.