I knew that I wanted to do something sciency so I took maths, chemistry and biology at AS because they were my favourite subjects. I didn't really want to do any other subjects, so ended up just doing computing as well, because I thought it might be fun.
It wasn't. I hate it. But now I can drop it, and I have a good combination of subjects for loads of different science courses. Honestly, just do what you enjoy, and it'll take you to where you want to be. Am I right in thinking that law doesn't really require you to have certain subjects? You could do biology, chemistry, maths and then english/history/a language or something? I think maths is quite important if you want to study medicine. It is quite a difficult position! Or maybe you could do biology and chemistry, and then two more essay-based subjects/ a language? I'm not exactly sure on the entry requirements at university... whether maths or physics is essential or not.