As everyone else will say, it is most important that you choose subjects that you will both enjoy and succeed in. You won't succeed if you don't enjoy it, and there is no point doing a subject just cause you enjoy it if you end up getting ***** marks - you are getting into a competitive market.
However, if you are good at maths and physics, it is well worth considering them over anything else. They will be helpful when you are studying for your ATPL.
Chemistry is hard. It was generally considered the hardest A level at my school at the time. Interesting though...