Thanks all for all your advice and experiences.
Fennel, he doesn't really know yet (I've been looking first because I don't want him to get over-excited about somewhere that just isn't an option). But we have been asking him about it. He is VERY keen on maths and science and is very good at both. His main interest is going somewhere where he can do lots of exciting things in these subjects.
But I don't think he is able to envisage the other bits: what it would be like going somewhere where none of his friends go, travelling, social and personal wellbeing, being able to mix with lots of different people, working on the things he's not so good at rather than just focusing on what is easy for him or he has a talent at.
He's a high maintenance boy: needs firm boundaries and lots of food/exercise! He kicks off A LOT and has only just recently begun to find other non-aggressive ways of resolving conflict and frustration...
I really am an old hippy so it is odd to find myself saying these things about my own son, but I do think he might be happiest in a pretty traditional old-school school where competition, clear discipline and lots of sport are the order of the day.
As for DS2, well we'll have to make a completely different decision about him later as so far he's chilled, easy going and mostly into painting .
no, I don't plan to hothouse him AT ALL. He wants to try doing some practice papers, partly because he like to challenge himself. In fact he just did a bit of a maths 11+ paper this morning before school and got 15 questions right in 15 minutes. So tbh I think he will be fine on maths and VR and NVR with a bit of familiarising (and algebra practice ). But if there's an English paper, I dunno. That we would have to bribe force cajole him to practice. But that is not because of a dislike of reading/writing, just the boring crap they do in school literacy sessions...
And I think we are going to play it fairly low key: we might take a look at the GS, we might do the exam just to see (no pressure) and make our collective minds up later.
The travelling bit, lack of local friends etc still bothers me but I will think on it some more.