If you are wondering what will best help you with your flying course, I'd have to say the Physics GCSE myself. (Mind you, I say this as a Physics graduate, so perhaps I am as biased as the mathematicians above ;o) A-level maths as you say, is generally just the application of calculus to problem solving - which is not required for your flying studies. GCSE Physics however, is equations, basic proofs and graph reading, with a bit of instrumentation and electronics thrown in. This is basically what your flying studies will consist of. Whenever I am asked to describe the content, level of difficulty and volume of info to learn at ground school, I always think of GCSE Physics.
However, A-level maths would look good to a future employer, so I recommend doing both if you can.