Warwick take your A*s for granted - STEP is their distinguishing factor. If you want to get in, you will almost definitely have to take it (or some equivalent exam).
They give offers that have either STEP or AEA grades, but I don't think they offer purely based on A levels. Fact is everyone going to study maths at Warwick will be getting A*A in maths and F. Maths at least and so they need a way to differentiate between candidates.
Ahh, at least they only want a 2 on any STEP paper, rather than 1's on Step II and III like most Cambridge colleges prefer.
My dilemma really is whether I apply to Oxford or Cambridge, whilst I know I will probably apply to Warwick and have to sit STEP, I have no idea whether I should therefore have a go at meeting a challenging Cambridge offer or spreading myself over Step I for Warwick and the Oxford MAT?
In some years Warwick have accepted a number of people who missed their offers due to STEP, but what I'd say is that getting a 2 in any STEP paper isn't particularly difficult with practice. The Warwick offer shouldn't influence your decision between Oxford and Cambridge.