I'd say pretty much no chance for Thornden. To the best of my knowledge, no one has gotten a place there over the past 4 or 5 years who lives outside the catchment- legally, anyway! There's a fair bit of 'renting in' for 6 months goes on. DS2 mate, one street out on the western fringe of the catchment, didn't get a place (is now at Mountbatten, Romsey, another good school)
We moved to get the DSs into Thornden. I am aware that in the recent past, DCs from a fair way away have succeeded in getting into Kings, because, of course, much of its catchment is fields! I have no idea what the change of catchment boundaries will entail. Winchester has been seriously caught out this year with YR DC- nowhere near enough places for them, I gather, and I believe Westgate Comp is planning on opening an on-site feeder primary school...?
I also looked at Kings to compare with Thornden. I was quite surprised at how different they both felt (and honestly, having loads of mates with DC at Kings can say...)- Kings felt like a grammar school (having been to one myself!). Everyone knew their exact ranking, be it 1A, 4C etc. Thornden claims not to set at all! Which it sort of doesn't. They set for Maths in Y7, then MFL (clever DC get a second choice in Y9). Other than that, they appear to be taught in reasonably wide ability groups BUT the way they do it is sort of selection via exam route, ie double v. triple science, foundation v. higher GCSE courses. It seems to work in that Thornden has the best academic record of any comp in Hampshire, without the success/fail of 'setting'.
We chose Thornden because of this. DS1 is reasonably bright but DS2 isn't. He doesn't need to be in a class labelled 4C for English, for instance.
Kings has a more mixed intake, socially, too (though not very ethnically diverse, as Winchester isn't!). Some friends' DC have spoken of a bit of an us and them feeling there- the MC DC with doctor parents versus the DC from the big council estate, Stanmore. Thornden is very MC (practically non-existent FSM) which won't suit everyone. It has a fair number of non-white DC from MC Asian backgrounds!