I am in year 13 and I do psychology. Honestly, I enjoy it so much and I find it helps me a lot when it comes to empathy and altruism. At times, it also links in with biology. I also did maths at AS but dropped it at A2. Maths was something else- I enjoyed some parts but when it came to exams it stressed me out so much. The uncertainty of AS results day killed me, that's how I knew I had to drop it.
What I'm really trying to say here is choose the one you enjoy the most and know you are going to do well in. All universities apart from Cambridge (who want three of maths, chemistry, biology and physics) require only chemistry and biology. If anything, doing something other than the standard chemistry, biology and maths sets you apart from most medical applicants. Good luck though, hope the choice you make is right for you