It's unusual for airlines to demand A-level or equivalent passes in subjects, and so I wouldn't worry too much from an airline point of view. From a personal point of view, I took A-levels which I knew would make the training a little easier.
The ATPL exams are possible just by 'learning the question bank' without much prior knowledge. However, I wouldn't recommend this, I think it's much more sensible to have an appreciation and understanding of the subjects involved. Not to mention you're paying in the region of £4K just to sit the theory exams alone, you might as well make that £4K count for something!
If you agree with this train of thought, then I'd say the obvious subjects are Maths and Physics, there's a fair amount of both in the ATPL's (particularly Aircraft Knowledge, and parts of Meteorology). I'm not saying you need Maths and Physics to attempt these subjects, just that it'll help you to learn them a little smoother without having to sit on Google reminding yourself how to multiply fractions and such!