No one does STEP before the interview. If you're that good at STEP now, then congrats, you're guaranteed an easy S, S in year 13 when you do take it, but Cambridge won't care much if you take STEP I and get a 1 in year 12. Of course it's quite impressive, but you could be doing much better stuff with your time - they'd like you much more if you'd taken the BMO or done some UKMT challenges or read a few maths books... the offer they give you is your chance to prove how clever you are, that's not what the interview and personal statement are for. They're for showing your interest and motivation in the subject.
Trust me, read some maths books, do a UKMT challenge and maybe BMO1 and BMO2 if you can... it'd be much more worthwhile than doing STEP. Put another way, they don't want to know you've been wasting your time on STEP when you're gonna do that in year 13 anyway and you could've been reading books and so on.