I made the mistake of studying A Levels which I thought would be useful, and didn't do as well as I could have done, so re-assessed and changed some subjects in my second year. I think that the ATPL groundschool teaches you everything you need to know for the exams - how well you do is mostly down to how much effort you are willing to put in. One person in my class left school at 16 and still did ok, although he stuggled getting back into revision after 6 years. As some posters have mentioned already it's the intensity and volume of information not the difficulty of it.
Davidson you definitely seem to have the motivation and I am sure you will get there - my point is that A Level subjects really don't matter, people just want to see you can apply yourself. I am certainly not the brightest tool in the toolbox but put my life on hold and slogged away for a 93% average - having done art based A levels. It can be done! Good luck