Do you think you don't want to do them because of nerves maybe? Most parents just want the best for their children so I guess from their perspective they want you to have the best opportunities in life (Not that having A levels/A degree will necessarily guarantee success - it's just more commonly found). If you try it for a few months and your parents can see you've really committed to trying and not given up without giving it a chance, they will probably be more understanding when you don't want to continue, obviously that wouldn't be ideal though as you'd be a year behind, so if you are 100000% confident then I would see about switching to a BTEC or getting some work experience through an apprenticeship maybe?
A Levels are a step up from GCSE's, you are given more freedom and it's about having the discipline to organise your time effectively. I had the complete opposite situation to you, my parents couldn't care less about my education, neither of them continued post 16, and never took interest into my academia. I just completed my Alevels and got A*AA which was a shock as I was predicted ABB and got BCCC at AS.
I was quite apprehensive about starting because I got 2B's and 5C's at GCSE and felt rather dumb compared to everyone else. But I have really enjoyed my A levels and really got stuck in with all the extracurricular stuff at sixth form, my teachers have been amazing, probably more so cause it was the first time I've had adults really push me to achieve my best. I'm sure once you get into the swing of it, you will really enjoy it all. Getting my grades last month was the best feeling. All the best with whatever you decide to do!!!