P.p.p.s i am an international applicants, and i belive A* in Physics and results from STEP II should cover the A in Math.
It's not what you "believe" that counts, it's what the university wants that matters!
If you had achieved 1,1 in STEP then Cambridge would probably take the view that that mitigates a slight underperformance at A level, since STEP generally provides a strong indicator of performance at Cambridge.
I'd suggest contacting Oxford first to see whether you have a chance as a post-A level candidate with your current grades, and if not then consider re-sitting Maths to get the A*.
If you're not confident of getting a 1 in STEP III then my honest opinion would be that you would find Cambridge a struggle.
But you should really widen your sights if you want a UK maths degree - somewhere like Warwick offers a perfectly respectable course!
A-levels: A*A*A The A*s must be obtained in Mathematics and Further Mathematics (if taken)."
Unless you have extenuating circumstances, I think that rules Oxford out. It's worth contacting admissions and checking, but if they say no it's really not worth applying. Ditto the 3 in STEP for Cambridge.
What about Warwick, Bristol, Imperial? Or one of the many other great UK universities for maths. Oxbridge really isn't the be-all and end-all, and you might even enjoy yourself more somewhere else.