If the top 10% from each state feeder school were selected it would be a start.
This is never going to happen and nor should it.
My catchment primary feeds into a very sought after comprehensive. More than 10% of children were getting L6 maths (under the old KS2 levels terminology); more than two thirds were above L5 maths. Parents move into these catchments because of the schools - and they are not necessarily very wealthy, but care deeply about education.
A few miles away there is a primary school with very different demographics in which less than 15% of kids get L5 maths.
Taking the top 10% of both schools would result in L6 kids from the first school missing the cut while low L5 kids from the second school get in. I can't see how this makes any sense whatsoever.
BTW I would happily campaign to get rid of state schools selecting on religion etc too. I find it disgusting that catchment schools select on religion - either potentially denying local children places and forcing them to travel or inflicting religion onto them. Very few countries in the world have state schools with religious affiliation.
It is just that, without an 11+, or equivalent unbiased exam.
If the exam is genuinely "unbiased", why do you want to allow the top 10% of all schools to pass, regardless of scores? Surely this is because you acknowledge that in reality it is always going to be biased - it is biased towards children from affluent, educated family backgrounds.