Two friends of mine came from Malvern, they said it wasn't a great atmosphere but it's probably OK. They seem to get quite good results.
I have one friend at Oakham, haven't talked to him in a while but I hear quite nice things about it, not sure how they compare resultswise though.
Ardingly I've heard is quite posh, but that's all I know about it.
And yeah, Sevenoaks has to have a mention. You'll get all the help you could ask for in terms of good teaching and being surrounded by other bright people. It's pretty difficult to get in for the sixth form though, it's practically like applying for university - exams, interviews, the works.
Good luck, wherever you go.