I lost a bit of interest half way through, so haven't read the whole of the thread, but, for what it's worth, here's what I think:
I went to a grammar school and did well, but I am against them. My DP went to a grammar school and hated it, he was good at English and not Maths and treated like an idiot, he "disengaged" and ended up going to catering college as he didn't want to do anything academic. My sister went to a comp instead (dyslexic and told not to go to grammar) and has done very well. I think that it is horrendous to split kids up at 11 and cream off some academic ones. It damages those that don't get it and it damages those that are not at the top.
"More or less" on R4 looked at the results for grammar school areas compared to the rest of the country and found that the bright kids didn't get better results than elsewhere and the kids that didn't go got much worse results. So, not an advantage if you are bright, but damaging if you aren't.
Where I live, we have a choice of 3 good comprehensives and I am very happily sending my DD to one of them. Admittedly, I am probably in the 'paid more for my house' bracket, but what about the excellent schools in central London now. I know that the Hackney Academy has done really well - not a leafy suburb is it?
If it was up to me, I would get rid of private schools and grammar schools. I think that if there were no option for the "privileged" then there would be a knock on improvement across the board as they engaged with their local schools. My cousins went to a dodgier comp because their parents are committed left-wing idealist types. They both got excellent exam results. I think that if all the wealthier middle class kids were going to the same schools as every one else, then the results of those schools would improve. But, it is possible to improve schools without this as well (eg Hackney), it just takes a lot more hard work!