As one with A-Level Maths and Physics, plus a degree in Aero Eng, yet still getting rejected by BA after FB, I'd have to come to the following conclusion:
Airlines want people who do not quit. Ever. If you think you'll give A-Level maths a go, then come out with a poor grade, that may count against you more than not having done it at all.
The fact is (and I may well be corrected here), most people who apply to be pilots are those that have done Maths, Physics, etc. at school. Many have a degree, and Aero Eng is a common one. If you turn up with Art, Geography and Music and a degree in Geology, you may just stand out from the rest, despite what the airlines might say in their literature. They want a broad cross-section of people, and are under increasing pressure to find such people.
Bear in mind you may/will need to pass aptitude tests. These will include maths tests, but believe me, they are nothing like A-level. More like GCSE on speed (and don't start doing that sh*t either...)
From what I gather though, what airlines want is a 100% total commitment to aviation. Go with your heart, do what you're best at. Get the best grades you can, apply to airlines and UCAS. Do a degree in whatever you enjoy most. Get the best results you can.
But if you want that airline job, keep that in the back of your mind, whatever you decide. When your friends are working in Tesco, work at an airfield. When they spend their uni grant on beer, get an hour or two of flying in (all airlines accept that school kids and students can't afford many hours, but if you have just a few hours over a three year period, it's infinitely better than nothing at all). When your friends do D of E at school, join the ATC. When they join the wine tasting society at uni, join the Air Squadron.
These are the things that count most.
So you do all these things and decide that maybe flying isn't for you. Then you can see how the airline recruiters might think about those that have never tried.
So you do all these things and decide that flying is for you?
Then you WILL make it. One day. Somehow.
There's a wealth of people on this forum that wish they'd started earlier. You're in a most fortunate position. Make it count. And the most sincere best of luck to you.
WGW