- Be conservative with your temperature ratings. A -3C bag rarely feels comfortable and warm at -3C ..
- Down vs Synthetic. Down is a lot more expensive and will last you a lot longer but obviously up front cost is the downside.
- Weight and packability. Weight isn't too much of an issue for me (you want to be warm after all), but packability may mean a large synthetic winter sleeping bag will force you to bring a larger rucksack. If you're making a big purchase it could be worth bringing your camping gear packed in a rucksack to a shop and see if it'll all fit together.
As for brands, PHD and Cumulus have both been good in my experience for down, however I'd probably class myself as a 'heavy user' so £250 quid per sleeping bag is less of a bother. Snugpak do cheaper synthetic options (with a great warm hood for the winter bags).
For UK conditions I'd tend to say -2 to 5C counts as 3-season, -10C counts as a winter bag, if you find yourself camping at campsites then you might want a cheaper bag as you'll (usually) be more protected from the colder weather.