I appreciate your enthusiasm for mathematics, but some of my friends got A* in GCSE Maths and got C/D/U from january exams in C1. It's a big difference from GCSE. You really need to grasp every concept and fully understand it to get a good mark. That's just at A level. I can only imagine it's even more important at degree level.
I think you're taking things too fast. I think that if you get the A* in Maths GCSE. Do the AS Maths For Year 12. See how you go with that, if you end up getting top marks at the end, self teach yourself further maths and maybe even additional further.
Also if you find it hard to get the mathematical information from the words they give you it'll only get harder as you progress through. You really need to think about the amount of work you will need to put into it.
You'll only really realise it if you do A level maths, but GCSE maths is incredibly easy, if you're getting Cs right now. That's probably not the best indication. If you can turn it around and get an A* in August, then you should be able to prove to your Head of Maths that you can do A level maths. But you should take it slowly, I myself do Maths and Further Maths (AS) and I found the jump from GCSE to A level slightly harder than what I thought it would be, I got ABB in my 3 maths exams in january and considering I got an A* in maths GCSE, from that alone it might be harder for you. So i think you should take it slowly and gradually pick up the pace if you are dealing with it well, but don't go full on OU maths course at the end of year 11. Just take it easy.