In reply to iksander:
I think you've got to consider three things: How often the it's going to be used, who by, and how much weight/bulk are you prepared to carry?
If you just want an emergency thing the blizzard vests are very good for sticking on other people - think hypothermic person wearing shit clothing.
The blizzard bags are also excellent in terms of warmth for bulk - but they do weigh a fair bit to be carrying around all the time though and I'd be a bit reluctant to break open the vacuum pack unless I really needed to.
(Also, if somebody has to wait with a casualty for a long time they're likely to get cold sitting about doing nothing)
The Superlite 2 person bothy is excellent -especially if there's two of you. It's very light and compact. Bit small, but that's the point I guess, more squashed = warmer! ...However, I wouldn't want to pay £70 for it!
I've always found regular bivy bags to be pretty cold compared with a bothy bag/KISU.
A cheap bothy bag might be your best bet - you can probably find one for not much more than £20.