Don't forget that the days are so much shorter, otherwise it would be a lot more bearable. I've only ever done it once - an Xmas backpacking trip to Dartmoor. Temperatures were predicted to be above freezing so I took a bivvy tent and Buffalo pile bag, plus my 3 piece Buffalo suit to wear in the bag. But it froze to at least -10C every night, the water in my bag froze solid as did my boots. 15 hours at a time is a long while to spend shivering through the night, I packed up and went home at the first opportunity!