Erm, its not that bad, just do some work at the end. You'll find it comes back very fast. I did no maths my entire GAP year, and looked it over quickly a few days before starting uni. I've started, disaster hasnt struck. A lot of uni stuff is brand new, and actually doesn't build on A level. The stuff that does build on A level is staggered, so you have time to revise it while your at university.
Four modules (I guess other universities are similar): Analysis, Algrebra, Stats/Mechanics, and one more which is mostly just further maths in more detail (1401). Analysis is completely new, and so far hasn't included anything you'd know from A level, apart from algebraic manipulation, but you won't forget that. Algebra has included some stuff from A level so far (only matricies, and some people might not of even done them anyway). Stats/Mechanics is taught as if you have no previous knowledge. 1401 has included a lot of A level stuff, but then a lot of A level stuff is being retaught in this module too (albeit much faster).
You have a lot of time to revise your A level stuff, if you make time to do it. I'm in week 4 at the moment, and if I joined the course now I could probably still catch up. Don't worry about it.