nikkershaw -
Yes, but you're forgetting that the point of schools like Sunningdale isn't academic 'excellence' (whatever that is) but training in the habits, attitudes and expectations that mark you out as a member of the ruling elite. That, and the access they give to the 'right' schools, universities, clubs, friends, etc. that are necessary to take the next step on the ladder to consolidating power and privilege.
It says it all that that the keys to getting accepted into Eton, Harrow, Hogwarts, etc. were the handshake, the confident gaze, and the ability to talk about great 'chaps' who'd been to the school before you. The academic programme is meaningless compared to all this class-power and class-culture flummery. And remember, the headmaster almost let it slip himself when he remarked that Churchill might have been an illustious old Harrovian, but he was rather thick academically.