In reply to drolex:
I was previously using a Rab -18 rated exped bag in the UK (because thats what I owned other than a light summer bag) and last year switched to a much more sensible -9 Western Mountaineering one. So happy with it - it really is such a nice piece of kit. Not cheap though, but probably cheaper than PHD.
I really think top quality down is necessary in bags for cold temperatures, otherwise you end up carrying huge stuff sacks and weight for the same temperature rated bag. Fit/cut is also important - you don't want all the heat escaping, or warming up superfluous bag around your legs etc, but bearing that in mind, any of the top producers filling bags will 850+ down and a high quality light weight fabric should do the trick - Rab, ME, PDH, WM, MHW, etc.
If your car camping, you could always consider a synthetic bag? Better suited to cope with the damp British winter (and summer) climate. It will be heavier and bulkier for the same temperature rating, but that might not be an issue for you. I saw recently that the latest Mountain Hardware bag won some awards.