I'm sure that if you're doing A-Level Maths and e-mail Warwick, they'll be happy to make a conditional offer (e.g. AAA with A in Maths) otherwise they're pretty strict on GCSE Maths at Grade A (presumably to show you can keep up with the stats modules). Warwick aside there are tons of other great business unis that will take you with a grade B in Maths (e.g. Aston, Bath (No1 in UK supposedly) Manchester, Birmingham, Lancaster, Loughborough etc!). Hell if you're doing A level maths and have grade A you can go to LSE which I believe to be the best Management degree in the country (if you can tolerate London living costs).
To summarise, yes, you can get into top Business universities with your grades (as long as you can get AAA-ABB).
Good Luck!