Hi,
Airlines like to see you have taken education further beyond what is required, gone up a level so to speak where the work load has increased and it is more in depth with a greater volume of work! Also A-Levels are recognised as the step up from GCSE so it seems logical they are set as the bench mark for qualifications.
The ATPL ground school is very in-depth, it involves commitment, in fact a huge amount of commitment due to the shear volume of work, more then GCSE I would say!
It may be work checking to see that the training you are doing as a apprentice may give you some form of A-Level equivalents' if so airlines may accept them.
Cadet places are very very hard to come by and the completion is fierce you could always carry on with your training and use your skills to save up for your flight training and do it that way. Or you could head back to college to do A-Levels, but unfortunately A-Levels are the bench mark when it comes to education requirements for these schemes!
All the best though