The first thing you need to do is figure out roughly what sector your back-up plan is going to be in.
As said before, you don't really need to study anything outside the normal core curriculum to be a pilot. Ideally you should aim to get a good assortment of different subjects at GCSE. Don't try to specialise yet. Wait until you study towards a degree for that. The degree you want to do will in turn decide what A Levels you should do.
The key advice, however, is have a good backup plan. DON'T count on being a pilot for a living - this would apply just the same even in "the good times".