Have a look at Alpkit - they are the benchmark for decent quality down bags (much more efficient for weight/bulk than synthetic) in price terms, and they've just had new deliveries.
However, Black Friday is coming up (I work part time in retail, and I hate it), but there will probably be deals out there, so do some homework on what you want (minus 12 temp rating sounds about right), and then look. Rab, Mountain Equipment, Mountain Hardwear (for synthetics), are all widely available, plus Robens, Montane, Jack Wolfskin, Mammut and even the occasional TNF are all around. Buy the best you reasonably can - its worth every penny, and its worth going into Go Outdoors or Cotswolds just to see if Rab, ME or MH has the right fit for you.
Go Outdoors, Cotswolds, Trekkit (I got my ME Lightline from them, so trustworthy), Gaynors and Blacks will all have deals, although whether they will have what you want is another matter! And dont ignore things like closing down sales - I got my ME bag half price from my local Blacks when it closed, which saved me £140.
As for Snugpak, I've owned one for years, and yes, knock 5 degrees off that temp rating. I'm not buying one again, if only because there is so much competition out there.