Hi,
Well done on your brilliant maths grade!
I did Maths at A-level and for AS they really ease you in, the content in pure maths for the first two modules feels very like GCSE. Before we started A levels properly, our teacher made sure that we could get almost full marks on a GCSE paper to make sure we had all the content from the previous years before starting anything new.
Your AS level will probably be made up of pure maths (which is what you will have done at GCSE - all the good stuff like algebra, graphs etc), mechanics (basically a bit of physics - pulleys, forces etc) and stats (probability, distributions etc). It’s all quite nicely organised into these 3 groups!
AS level I thought was fine - the pure content is really interesting, mechanics and stats was a bit trickier as I’ve never been particularly keen on physics. A-level was a step up, the content was trickier but it’s still a brilliant course. If you love maths I’d say go for it - your GCSE shows you have the skills and ability to make it work! Whatever A-level you pick it’s going to be tricky so choose the ones you will enjoy putting the work into.
Hope this helps!