I don't see any reason why a local college/school can't sign you up for the exams though you will have to pay for them. You would of course have to teach yourself the material which could provide difficulties, not to say it isn't doable, especially if you take the full 3 years of your apprenticeships to learn the material and revise it along side your apprenticeship. It of course depends entirely on your academic ability, but it is definitely possible and all of the information you could need for the exams are available (at a price usually). It does seem like a very good idea to pursue A level qualifications along side your apprenticeship, definitely shows commitment and dedication, if I were an admissions officer at a uni you applied to I would be very impressed with that as you have gone far beyond what you needed to do.
If you're very good at maths, you may want to consider doing the further maths A level as well? I realize you may not want to as you'll be taking on a lot but when I was your age I didn't realize further maths was an A level I could do until I visited open days at my college. Just want to make you aware of it as a possibility if you're very good at maths.