Thank you, Happygardening! 
My children are still of primary age so we haven't reached the stage of GCSE and A'Level so maybe I'm being clouded by what I want their childhood to be about. The local catchment was one of the first 'Beacon' schools way back and gets ridiculously high SATs results but SATs seemed to be everything to the HT and, it seemed, to ofsted. The school also had two AST teachers on staff so I'm not sure I could be accused of paying for better teaching either.
However, I wanted the broadest possible education for my kids, especially DD1 who was labelled gifted at 3yrs and certainly is exceptionally bright. She has just started Y1 and is working (academically) at the level of a 10yr old. However, school whilst stretching her in that way also still treat her like the 6yr old she is. She still needs to learn the social stuff and her favourite part of school is art. As I said yesterday, I wanted her esp in an environment which would allow her to develop other talents and not see her as just a very bright girls. Having taught in state secondaries I have seen too often children like her forced into doing an excessive amount of academic work and told they can't do Home Economics or food nutrition or art and they need to be doing further maths etc. I simply didn't want that.
So no, I am not paying to ensure the highest academic results as, like Happygardening, I'm sure my DD1 would achieve wherever she is. I'm actually paying to ensure that's not all she gets.
Oh and obviously for somewhere that would offer lots to her sister and brothers. 