meb, my order would be
North London Collegiate
Haberdasher's
St Helen's
(that would also be the A level ranking from best to worst)
If you are into London itself then St Paul's Girls would be up there at the top or second. St Alban's - the private one, St Margaret's Bushey, Northwood College and Royal Masonic may be if they're not quite in the league of those at the top of the list. I've probably missed some goods ones out.
My daughters went to Habs and NLCS at 5 and 7 respectively but about half the entry is from state schools at 11. If you want the cleverest children you'd be stupid as a school just to recruit from the private prep schools.
We were warned for years (the schools are good) that daughter 1 might not pass at 11 and she did a lot of exams without any pressure at 11 but in end passed for Habs where she already was anyway. She's slightly dyslexic and did much better in the seniors where the subjects themselves rather than the basic handwriting and spelling come more into the fore.