The HCS GCSE results look pretty impressive on paper, I must say! And yes, they are selective, though I don't know what exactly they select ON!
I went to an inter-schools cross country meet there a couple of years ago when my DS was in a local state juniors, and I have to say the site is very impressive- acres of open ground, trees, all very nicely kept. The junior school is very new, built as a result of the merger. I know of a few DCs from my DC's state schools who went there for secondary, and one, a DS, Y5 who has just gone cos his mum thinks he needs an academic kick up the bum before state secondary!
I gather DCs go there from ALL OVER- you'll see them in their purple uniforms waiting at bus stops in Winchester at 7.20am, and, whilst at the end-of-school cross country thing, there were DOZENS of buses lined up on the driveway!
It is my impression that the school expect you to at least pretend to commit to secondary school there- though as someone just said, many leave. I am surprised it is seen as a prep for places like St Swithuns, though- I'd've thought The Stroud in Romsey or Tywford Prep would be more appropriate, but that's just my opinion.
KES is where the Winchester consultants send their DCs when they just can't quite bear to send their DCs to either of the 2 outstanding comprehensives in the town, but can't stretch to St Swithuns or can't get their DS in to Winchester College!