At GCSE I got an A* in maths. Honestly I didn't find the transition between GCSE and A-level difficult, and many of the concepts in C1 (easiest module) are very simular to GCSE, so it felt more of a review. The style of questions are slightly different though, as the questions are more focused on functions and manipulation of algebra, and less of basic computation of numbers. At AS I was so excited to learn more complex maths that I started to learn C3 and C4 (A2 modules) before my AS exams, but this lead to me neglecting C2 and S1 (Harder AS modules) past papers, which resulted in me getting a B (2ums from an A) at AS. Just a heads up in case you make the same mistake as me.
This year I got an A* in maths, and an A* in further maths (I took AS and A2 this year) so even if the first year doesn't go as well as you would like, you progress naturally through the second year and the course doesn't seem to get more difficult, in my opinion.