That is a really complicated system that you have there lol, and after reading that through... I don't understand why any school would have a system like that. But never mind lol.
I wouldn't bother sitting maths again.... No point. It's not exactly anything you don't already know, so unless it's a university entrance requirement, then there is no point in doing it. I mean, if you ask anyone who's done AS maths to do GCSE, they'll get 100% easily. There is no point, in my opinion. AS maths builds on GCSE knowledge, so if you can do AS, you can do GCSE. It is assumed.