Unless you have other personal reasons not to, you might be better off giving one of your choice slots to Oxford or Cambridge since slightly lacking A level/GCSE grades for a candidate that is actually very good are less of an issue there. Places like Durham/Warwick etc wont give you an interview and will rely solely on what is on a piece of paper. Of course it's not exactly an easy route, but it's fairer since your mathematical ability will be thoroughly tested and if you deserve a place, you'll get one.