You'll need GCSE Cs or better in Maths and at least one English, but check. Clearly you need to take advice from your tutors on choice of A-levels - I know from my daughter's experience that Maths, Physics, Chemistry and Biology are all hard work!
You need to realise that to pass for Observer and Pilot you'll need very good mental maths skills. If you are struggling to get a C grade at GCSE then I'd question your chances, personally.