JWF.
I dont see how airlines will use academic filters for recruitment purposes. I too do regret studying harder at school, but I am not going to let it hinder me, any pilot with a desire to fly would not let it hinder them.
After all, if you can get through the hurdles of the ATPL course, you can do anything! GCSE's are irrelivant.
Even if it does come to the point where one does need GSCE's on top of an fATPL or ATPL they are not hard to get. Night classes for 3 weeks and an exam will get you a refresher in GCSE maths.
Cheers
Savage