The jump between GCSE maths to AS maths is small, there's no "holy **** this is a real step up" the second you start the course, as with some other A-level subjects. In my opinion the maths A-level courses are structured really well to gradually break you into the harder maths. Just aim to ace the AS, as A2 is a step up and is a lot more challenging, so good scores in the AS modules will mean you have a much easier time when things get trickier.
You'll be fine, my school didn't even let me sit the subject despite getting an A in GCSE as I failed an entrance test by a mark, so their logic was that I was incapable of getting even an E. I was taught independently, and went on to get an A at AS, and having just sat my A2s am on target to get an A overall.