I thought all subjects would be linear next year, it just means that if you choose to do an AS level that it doesn't count towards the a level. Idk if it is or not,look of the DfE website
I do both. C1 is not a big step up from GCSE, but it will get a lot harder in C2
FP1 will depend a lot on how you understand maths, there's a lot of new abstract content that you may find hard to wrap your head around. It's a lot easier if you've done further maths at gcse, so if your school doesn't offer that, self teach some of the bits, it really bridges the gap