I cannot help but notice that many of you are trying to argue the toss as to why Oxbridge should accept your kids and shoehorn your idea of a good candidate should be into the general discourse.
Oxbridge don't care about about: D of E, rowing, being a prefect, volunteering in unrelated roles, your sport (unless your UK/world ranking in high), indeed any extra-curricular stuff unrelated to your course.
Oxbridge is no longer a finishing school for the rich. It hasn't been for a while. Like, I know they churn out loads of prime ministers, but they do need to do useful stuff too, like discover stuff and make things. They can't do that if they fill their colleges with Norman Rockwell types with good orthodontics and shiny hair, who aren't actually exceptional. What is exceptional?
You need 9s, A, A^ at GCSE. Some 8s are Ok and you'd better be special in your chosen subjects (like rainman special) if you've got 7s or go to the type of comp where the teachers lust over pritt sticks and the kids use the empty cases as missiles once they've rubbed them all over the seats. You need to be top of your cohort, no matter where you are educated. So if your kid is at the top, or very near the top, that's good. If your kid gets the same grades as everyone else in their cohort, that's not special. Even if you paid for it.
What else? Be able to talk about your subject, or your chosen area within that subject. Not with aplomb. Just talk. Hesitantly, looking at the floor, stuttering, pausing. Waggling hands, sitting on hands, without engaging head tilt. Just talk and then when you're offered new info, or asked about an alternative perspective be knowledgeable enough to assimilate, or make links and talk some more.
So: Be smarter than your peers, be knowledgeable about your subject and be able to demonstrate this to the extent that your interviewer feels that you'd turn your Oxbridge place into something the university can put into a journal or into someone's arm