@omgitcantbetrue I am glad your dc was happy at the prospect of moving! The head wouldn't necessarily know, in my experience, by the way - though it is less relevant for you now! Also you said upthread that you underachieved - a pp has suggested a 1:1 tutor for a bit and this may well be a really good idea if you get a good tutor with dyslexia experience because a lot of academic success comes down to learning good study skills and "know how", if you get a tutor who is good at this then it will make your dc's learning easier now and in the future right through further education, whatever school he is at. Good luck!
@WombatChocolate just to answer you from last night, I don't think that the fact that the schools you mention get a lot of applications means that all the parents' dominant motivation is to be part of an exclusive academic elite. Some may be motivated by wanting to join a social group, I guess, but most ime will have the motivations I mentioned upthread (facilities, opportunities and expectations, small classes, few discipline issues). I would be surprised that if any greater London selective schools had one or two applicants per place - which ones are you thinking of? Can you link the stats you are referring to for all the schools?
If I had a choice of schools for dc I would want them to be at an academic school as I think they'd be happier, and I think they'd perform better under pressure, just because of what they are like, but I wouldn't necessarily choose any of the schools you mentioned because of their culture - there are others which are more quirky as well as academic (imo) which I think my dc would be happier at. Wanting an academic school isn't the same thing as wanting to be part of an exclusive group.
Henrietta Barnet is a selective grammar (I think? It used to be) so that would explain the high number of applications there.
This is just based on my personal experiences, I don't work in education.