Mike, down's great when it's cold and dry - alps, Himalaya, Andes, my kitchen - not so hot in UK damp-cold conditions, unless you're in pub/tent/house/hut etc. Water turns it into a soggy mess. Don't know about the specific one you mention.
A duvet-type jacket using synthetic fill is going to be better in the UK. GoLite do a couple, I think Snugpak do one, tho I've not used it.
Dave, sorry mate, I guess it is kind of geek talk. I'd summarise it all by saying: Britain - synthetic filled insulated garments good, down, not so good cos of the wetness.
Sigh. Anyway, I'm on holiday now! Hurrah. See you all in ten days time.