I'm planning to do comp. sci at Cambridge. I sent their admissions team an e-mail regarding the 'usefulness' of a STEP exam, even though it's not explicitly stated in their standard offer. Here's what they replied back:
(when asked about taking STEP 1 as an amelioration for lack of A2 FM):
" It would be very unusual if we included STEP 1 in an offer for CS/NS or CS/75% just because he says that he's taking it. There's an outside chance that we would use STEP 1 to make an offer if we had strong doubts about his Maths from interview or CSAT, but in that case the STEP would mean that he gets an offer rather than not getting an offer at all, so it's still in his interests to take STEP 1."
followed by
"If we had doubts about his maths in interview or CSAT, then doing STEP 1 could make up for it -- in that sense, doing STEP 1 would count in his favour and possibly make him more likely to get an offer.".
"STEP 1 doesn't fully make up for only doing FM to AS level, but it certainly helps -- both in terms of likelihood of doing well enough at interview and in CSAT to get an offer and in terms of how well he'd cope with the Tripos once admitted."
This was answered by Dr. Fawcett from Churchill college.
Keep in mind, this is for comp. sci, not pure maths, but I still feel as though this gives a good insight into how some universities view it.