Erm, it's unlikely tbh.
An AS is assessed out of 300 UMS points (at least a 3 Module AS like Maths is). If you're only half way through C1 with 10 weeks left before the exam (or something like that) I really doubt you're going to have time to cover C2 and S1/M1/D1 to a good enough grade to get a B or even a C, since you'll need approximately 200 UMS to get a C. If you can storm through C1, which should be easy since it's essentially just Higher GCSE maths with Calculus thrown in, ace that (90%+), you could maybe then cover C2 and S1/M1/D1 to C level, but C2 is where A Level maths actually starts, and S1/M1 have a lot of content.
Ask yourself if there is any actual benefit to doing it a year early. If you end up doing Maths A Level (either Further or Regular) all you've really done is cover it a year early. I personally wouldn't be happy with anything less than a B in an A Level, so you're selling yourself short by going away with a C at AS. If you don't plan on doing Maths at A Level, you may have an AS under your belt, but it'll count for very little in the long run.