The FMSP is a programme mainly for students whose schools don't teach Further Maths. They offer masterclasses for help on STEP and other such activities - I'm not too informed on them.
Super curriculars aren't actually a huge deal. Of course, you must show that you're enthusiastic - that much is obvious - but, mainly your offer rests on your solid AS grades and then your interview.
Cambridge will basically give any candidate with excellent grades a chance to prove their prowess in an interview.