no direct experience of SEN at Oxted, but the pastoral side there is vv good. Some classes/children do have TAs in class with them, but don't know how they are allocated/assessed. There's a VI unit attached to the school, and there is a fair amount of specialist equipment available.
Re less academic children, as it's a big school, there are lots of sets for subjects, so you don't get the same situation as in a smaller school where different abilitites are lumped in together simply to make up a full class. That has benefitted us at the more academic end certainly.
Best bet is to ring the school and speak to them, even get down for a visit, as IME everyone has been incredibly helpful.
Only other thing is that it is a big school, and is busy and noisy, so depending on your DS's actual needs (if he needs space and peace for example, then the corridors aren't ideal) there might be some issues, but not any more than in any senior school I'd've thought. Re being socially isolated, because there are so many pupils, "different" children don't stand out quite as much as in a smaller school, and there is a much bigger range of possible friendships. Loads of sporty boys, geeky boys, slacker boys, quiet boys, arty boys, musical boys.
Good luck - it's nice down here!