I live in Kent where we already have the grammar system.
DS1 didn't pass the 11+ , education didn't click for him until Year 10 when he started to specialise in his subjects. He went on to do well at GCSEs and then transferred to,the local grammar for 6th form. He's going to a Russell Group university next week to study Chemical Engineering.
Although, on paper, his non-grammar school wasn't high achieving, it was the making of him.
I think that it doesn't necessarily matter what kind of school a child goes to because in the end everyone finds a place.
Equally, he told me of many students who had passed the 11+ and been tutored to within an inch of their lives to do so and then post-GCSEs didn't put the work in so,we're asked to leave the 6th form.
I grew up in London and went to a comprehensive, it was awful.