My two penny's worth… 456 & Soul are not really poles apart, they're both long travel steel HT's, one certified to 140mm the other intended for up to 150mm. Taking a 16" frame for example, even geometry is very similar – 0.25" in the TT and 0.5 degree in the head angle, similar chainstays, so really very close in concept, until that is you come to price…
And £300 difference is a fair old whack in anybody's book, but then as somebody else said on another thread… "strong / light / cheap – pick two…" Not sure where the 600g came from either, as going from their websites, it appears the difference is more like 1100g (not exact as on-one don't specify a size for their 2495g frame).
But… you appear to be prepared to pay a reasonable amount for your new HT, by that I mean you don't appear to be working to a really strict (lowish) budget, so why not go for the Soul? You certainly wouldn't regret it, but equally if your total budget was less than half of what it probably is, then I'm sure you'd be delighted with the 456.
Another way of looking at it is Soul with average forks & wheels or 456 with mega forks & wheels, then if after 12 months you really must lose that kilo, then sell 456 frame for 1/2 to 2/3 what you paid for it and buy a soul, transfering everything else over… or if you've fallen in love with 456, well then, happy days.
And finally (sorry for the novel) since you have a nice FS already for the big stuff, how about a 100mm travel HT, since riding on the Ridgeway, beautiful as it can be, perhaps doesn't warrant 5 to 6" of full on steel butchness…???