I think based on merit, a majority of students accepted into Oxbridge have the capacity to also succeed at the Ivy League schools, based only on academic performance. People who go to Oxbridge are usually motivated, hard workers, enthusiastic about their subject and relatively self-sufficient anyway. Whether they did their subject at Harvard or Oxford, theyd still be successful. This potential being a vital admissions factor, theyd be likely to get in.
I believe the UK 'elite' unis are far more equitable than their US counterparts. A report recently published in the Economist described how you are 25 times as likely to run into a rich student as a poor one. Anyone know a comparative figure for Oxbridge?
Anyway, admissions factors with Ivy League are also based on 'legacies'- if you're parents went to Yale& co. you're 3 times more likely to get in. And yes it is true, your extra-curricular activies need to be amazing. In short, it favours the kind of child that has been ferried around from ballet class to chess, from piano lessons to extra maths all their lives. For the most part.