Well, first of all you have to get through the first few selection tests which don't really relate to any academic study you will have carried out and as said previously, the maths required at stage 1 is basic - GCSE standard at best.
If you get to stage 3, your educational background is likely to be discussed during interviews. In the end, the decision to recruit you or not comes down to a range of factors, so I don't think too much importance will be placed on having A-levels in this particular career. It could possibly show that you will be ready for the rigours of ATC college and have useful study techniques etc but I'm speculating.