I've yet to see any FTO website stipulate basic school qualifications. They all quote aviation minima required to proceed to the next level.
Those FTO's that do have aptitude testing I think include their own maths assessment, and therefore would probably not pay too much attention to your GCSE if you could pass that.
If the grade you got really niggles, why not do it again?
And I agree, school careers advice is the pits!
JR