In all honesty, dont worry too much about the GCSE's or, to an extent, the A-Levels / B-TEC / GNVQ / Modern Equivalent. Its all about how you apply yourself during your studies.
I would also reccommend a degree (if you lose your medical before you have enough aviation clout its useful). Avoid "BA (Hons) Airline Management" and similar as those degrees are largely a waste of time if you want to work in aviation (ironically).
Practical pilot maths: know how your 3x table works
