NLDNMum, it is very high, about 75% once you are in second round (speaking about the top North London schools).
Second round is mainly to see that the kids can operate in a group dynamic: listen to instructions from the teacher, have fun, be social and work in groups, and can participate in physical activities (dance, sports, musical games, a simple group science project). Things that cannot be tested on paper.
It is just a fairly low threshold screen. There is an actual interview, though: describe this picture, did you have a tutor, what is your favourite book, what do you do outside of school- so well advised to prep some answers for generic questions with some mock interviews at home, but equally important how to play with classmates.
None of these are serious pitfalls, either. Our kid made some really basic mistakes in the interview, and still made it through. Some kids that really shouldn't be in there made it through. So any social kid is in, even mildly on the spectrum will make it through as well.
All the kids I remember seeing in second round were part of the incoming batch, except one. The one kid that didn't make it had a really hard time with attention, like there are probably clinical names for it, those types of things they screen out, nothing else.