Well, if you’re taking maths and CS then, if you want to go to a top flight Uni, you will almost certainly need further maths. If you’re not planning on studying either of those at Uni then that’s fine.
Most Uni courses only require you to have an A level in the subject that you want to study and the others can be in almost anything else so why not just simply pick a subject that you enjoy, will work hard at (as you enjoy it) and will therefore get good grades in?
Oh, and did I mention you will enjoy it (which is what you should be looking to do with the things you study IMHO)?