Not sure that A-Levels are particularly sought after by the majority of airlines once you are qualified however some of the larger airlines may be put off if you lack A-Levels
and experience. At the end of the day anything that gives you an advantage over others is a bonus and A-Levels certainly wouldn't go amiss on your flying CV.
As for the ATPL theory I'm not so sure that A-Levels would be much benefit really. The standard of learning required to pass is GCSE A-C level. I only have GCSE's and although it was difficult at first to grasp some of the ATPL calculations I didn't really have too many problems. The main concern in ATPL theory is remembering the bulk of information.
If the aviation career didn't work out for whatever reason (medical or otherwise) I'm not too sure that A-Levels alone are going to put you much further ahead unless you have a degree under your belt. Therefore, if you're going to study for another two years you would probably be wise to extend to university afterwards. Bet you like the idea of that eh?
VFE.