From what I've seen so far uni maths is very different to A-level maths. If you can do well in FP1 and FP2 thats a good start, but you might want to look over some of FP3 just as an extra thing. I'm not sure if the spec is changing but FP3 had a good range of different things. Complex numbers, vectors, matrices, and proof by induction which is the 4 biggest chapters would probably all be worth a look.
I woudln't suggest doing anything until much closer the time. I didn't do anything extra until this week when I got a book to keep me occuppied over summer, and I there are people who do nothing extra and do fine. If you're looking to do STEP or AEA I'd start revision as early as possible. Nothing major, maybe looking through a few questions per week, but I didn't do any revision for them until like the weekend before and those 6 hours doing STEP were horrible!