I'll be honest here . If you want the best most optimum subject choice it's biology chemistry maths .people may disagree but go check your self .for medicine dentistry etc you typically need bio chem . If you suddenly switch out of no where and want to do something like engineering alot of unis only require maths . If you take another U turn and want to do law some universitys like uni of Bristol don't have a a level requirement but ofc a level English and other essay subjects would help . But if your extremely passionate about it you can do things on the side to prepare you more for a degree that doesn't particularly match your a level subjects ( If that makes any sense). Doing BCM for a levels will give you nearly an unlimited number options interms of degree choice and it's highly respected.