I remember when I was in year 9 (cue the violins).
The subjects that are important to flying are (or at least were) the compulsary subjects, such as Science (physics), Maths and English (maybe not as relevant). I had to choose 3 subjects, most of the options that I had, had no relevance to aviation so I choose two that I thought I would enjoy the most. I choose Spanish and Resistant Materials, and the third Geography, because people not in avaition, e.g. parents, seem to think Geography will help you as a pilot. It doesn't at all.
I agree with redsnail and ikea, do the subjects that you feel most comfortable with, though at least maths, physics and English should be included. I say again, the subjects which matter are possibly still compulsary and not an option.
Another factor is the path you intend to take. For sponsorship, airlines will look at GCSE's, so make sure the core subjects are there because sponsorships have a minimum requirement. E.g 5 GCSE's, with grade C or above in English and Maths.
If you intend to pay for the training yourself, then academic qualifications don't matter as much, though a recent GAPAN survey indicates that some airlines like a university entrance level of qualification.
I would get as qualified as possible so that you would be eligable for Sponsorship, but plan in the future to pay for the training yourself as Sponsorships are very competitive.
Hope this helps.
PA28.