Noblegiraffe, your point about Bristol is interesting and the contrast is what I believe you can see in the towns I used to, and now, live in.
Town 1, E Yorks: previously, grammar and secondary modern, grammar went private. Comprehensive very poor.
Obviously not ALL the local bright kids go to the private, but enough do (the whole top table in dd's primary class) to make a big difference.
If I only had 1 dc we would have done too, we just can't afford 3 sets of school fees. Many who do aren't that posh, it's far cheaper than comparable schools in the south.
Town 2, N Yorks: (which we moved to mainly for the schools). Previously grammar and secondary modern, both went comprehensive. Town now has 2 good schools, one more academic, though actually it's the former secondary modern that gets outstanding OFSTEDs - it does rural vocational stuff extremely well with a school farm.
DD is going to the former grammar and I was a bit taken aback by the relative poshness of the parents at the induction evening - I think a very large number of the parents, had they been in the other town, would have been sending their dcs private.
I do think the situation in town 2 is far better for everyone than it would be if one was grammar and one comprehensive. Town 1, not so clear. I don't know why the former secondary modern has proved so resistant to improvement.
However, where I grew up in Essex, the comprehensive I didn't go to because I went to a superselective 10 miles away, was really pretty good - it was huge so a good range of subjects, they got a couple of kids into Oxbridge and many into other good universities every year - it wasn't the 'secondary modern' described by other posters as inevitable once you have grammars, and there weren't that many private schools around, none within reasonable commuting distance.