I should declare my inherent hypocrisy as I have children at grammar schools but am intrinsically ideologically against them 
A few points: there is a vast chasm between what we want for society and what is for the greater good vs what we want for our own children (see above, my own hypocrisy). If anyone can solve that, they should have the gold star.
May's talk of bringing in new grammar schools is just a distraction: if we have money to spare, that money can go to improving all existing schools. God knows they need it, our grammar certainly does.
They are with other children who want to do well and not sneered at as some kind of weirdo or geek.
Well, just because it hasn't happened to you or yours (how lucky for you), you can't diminish the effects that being at the receiving end of such taunts and jeers has had for those of us who have been called swots, weirdos or worse...
I just hate the automatic assumption that when thinking about an education system, the wants of the most able (I use wants not needs advisedly) are the most important.
My experience with my kids' education is in direct contrast to Bert's statement above. Our overwhelming experience is that the system throughout primary school was focused on the low and middle attainers - the wants or needs of my academically able kids were not the most important thing to the school (I don't disagree with this on an impersonal scale). Focus and resources were given to "bring up" the bottom and high attainers were not stretched because they were already capable.
But as a poster said upthread, we're all fairly entrenched in our views and unlikely to persuade anyone else unfortunately.