Hiya,
I'm also thinking about applying to Cambridge for 2015 entry, although I'm not sure which college just yet! I have a 2.i from Oxford in Classics and AAA at A level in Latin, Greek, and French. I'm doing a Chemistry A Level this year and will then do AS Levels in Biology and Physics next year. Best advice is to make sure you have good work experience, have done lots of wider reading into areas of medicine that interest you, and that your personal statement and references are absolutely stellar. They do say that most applicants have AAA at A Level, but it's probably worth giving the college a ring and chatting to someone in the admissions office about it, as they'll be able to give you some good advice on your options and chances.
All the best!