Papa, this is just one person's observations of them. I only know two of the schools quite well.
KGS - co-ed, inner city. For some reason it didn't feel right to us, so we didn't look at it. (It's right in the middle of a pretty grubby ring road in inner Kingston whereas the others I've visited are all down leafy lanes or spread out on huge grounds.) Heard lots of mixed reports, including that it's more suited to girls than boys. Everyone says the Head is good.
Hampton - facilities to die for. They seem to invest lots of their income back into the school (new theatre, new sports facilities.) Massive school. Very strong extra-curricular stuff going on. Also seems to be strong on all subjects - great arts, sports and sciences, so suits all different tastes. New Head is ex-deputy. Feels more public school-y to me than the others. Head of Academics told me it reminds him of Eton where he used to work. I admire it tremendously rather than warm to it - there's something about it that makes me feel a boy has to toughen up a bit to thrive there. (Not criticising - DS has sat for it and would be really glad to be offered a place.)
Reeds. Great school. Only 800 pupils, so class sizes are smaller than Hampton, and it has a cosy, friendly feel to it. Less academic but still gets good results. Very strong pastoral care and superb facilities - sports fields and swimming pool on site. Great at getting all boys involved in sport of some sort. Much more expensive but also includes more in the fees. It's set down a leafy lane and feels like a safe haven from the big bad world, which may or may not be a positive. Has an outrageously wonderful new design and technology suite which had DS slavering like a dog. Mixed boarding and day. Head's retiring soon which is a shame as I think he's brilliant.
Both Hampton and Reeds look like they'd be incredibly good fun to be at, and friends with boys at both schools are very enthusiastic. Don't know enough about the others to judge.
Radnor House - never seen this one. It's new and small. Run as a business not charitable status. Friends with a DS there are happy with it.