How well you do in A-levels has absolutely zero relevance to how well you do in ATPL theory.
No offence but I think that's utter nonsense. More dumbing down of the profession and no small wonder that we are likened to bus drivers and paid accordingly!
GCSEs aren't worth the cardboard they're printed on. A-Levels are a basic standard of education. Do them for your own sake. Get an education! The ATPLs are hard if you actually want to learn the subject matter rather than simply parrot learn the multiple choice exams.
But seriously, if you think you can learn ATPL theory and ultimately pilot an airliner, with all the complexities that involves, you should be quite capable of getting a reasonable grade at A-Level. If not, well, I refer you to my first paragraph.