Dave
You need to strike a balancing act between (i) what you know will give you good grades; (ii) what you need as a basis for more advanced study (degree or ATPL); and (iii) what subjects you enjoy.
Increasingly school qualifications don't prepare you for anything - they are simply a currency that allow you to 'buy' a place at uni or the job interview table.
Having said that those on the other side of the 'table' respect some subjects more that others, as they are seen to be much more of a challenge. So an 'A' in one subject is sometimes seen to be 'worth' much much than an 'A' in another.
I'd suggest doing Maths and Physics - it's the conventional choice for pilots. They might make study of the ATPL theoretical knowledge subjects slightly easier but if you're a hard worker it won't make much of a difference if you have them or not.
On the other hand if you want to go to uni before becoming a pilot to study something like 'French Basket Weaving in the Context of Medieval Central European Poiltics, specialising in the Moral Aspects of the Reformation', then they might not be the most appropriate choice!
But the ATPL's aren't mathematically demanding. As others have said, they're broad but lack depth, particularly mathematical or scientific depth.
Tough call - as usual the choice is yours. But good luck with whatever you choose!