I think I will say that as a general rule if you can get AA in Maths and Further Maths, you will be very well set for the vast majority of maths courses, though don't expect it to be necessarily easy.
I suppose Cambridge (and possibly warwick) are the biggest (but by no means only) exception to this rule, if you are good enough for cambridge maths, the honest answer is, you should find it easy to get 95%+ in all the maths modules of A level, if this isnt true, it is unlikely you will be able to tackle STEP.
Again if you strongly disagree with me, then by all means reply, neg repping without any sort of explanation does not help anyone.