From my experience there is no need to do A-Levels in maths or physics unless you specifically want to and have a flair for both. It certainly won't stop you passing ATPLs' or bar you from a flying career. I didn't take maths or any science beyond GCSE.
The maths required in certain parts of the ATPL course can get tricky if you haven't got A-Level standard maths up your sleeve but it just requires a bit of effort and concentration to get your head round the principles involved. I found the standard of maths teaching at BGS was far superior to anything I ever encountered under the national curriculum!
Maths on the flight deck of a jet is mostly limited to some basic multiplication and division, and involves 'ball park figures' rather than precision. Personally I found myself doing far more mental maths under far more pressure during CPL nav in a light twin. Also bear in mind that while mental maths is often required in the air, the maths required on the ground in airline ops - fuel conversions for instance - will always be cross-checked using a calculator, as ball park isn't good enough in those cases! There really isn't too much to work up a sweat over. I find the hardest calculation of the day is trying to add up the duty hours of the crew after 4 sectors at 3am in the morning. I regularly resort to using my fingers at that point!
