I don't think GCSE subject choices matter much for medicine as all the core subjects are covered anyway. Do triple science if you can rather than double. What is more important is that you get good grades. At A level you will have to do chemistry, and usually biology. Some universities prefer you to do an unrelated subject for your third A level to show breadth of education. If you decide to do further maths you will have to do it as a fourth A level. If you do maths and further maths medical schools don't consider them as separate subjects.