Firstly, probably not medicine since people who want to do it will have decided before Year 12, at least in Year 10 (maybe 11 at a push), and will have worked towards it accordingly with some work experience. If you haven't got any relevant experience atm, you'll have to work quite hard to get a sufficient amount within less than a year and it might be a wasted effort. That said, if you haven't firmly decided on medicine by now, it's probably not the one for you and if I were you, I would stick to Biomed or Biosciences
Obviously, work as hard as you can- get in the teachers' good books and keep your performance constant
Maybe do a little extra, if you are capable sign up for Olympiads etc. and good scores here could indicate your strengths in sciences
It might be a little disadvantageous for as you don't sit AS levels and so there will be heavier reliance on admissions tests and GCSEs- ask around from current Year 13s to see what the admissions tests for the courses you are considering are like and how much prep they would recommend
Deciding on a college- have a look around when you go there and if you go to an open day for a particular course, pick up a college brochure for that course which will basically tell you about each college in relation to the uni in general and the course- people often pick colleges based on student intake and also proximity to where they will be studying
Personal statement- make it quite academic; focus on particular parts of different sciences, maybe throw in a couple of relevant books and try to do related EPQ if you can- you can talk about this
As for picking between Ox and Cam, just see which one you feel more attracted to. If you have strong GCSEs maybe Ox since they have always put a stronger emphasis on them