The DAS Parka is available in the UK and it a great "high end" synthetic that's been well proven. I have used one climbing as a belay jacket, I throw it on over the waterproof for belays. The cheapest synthetic option in the UK will be a Snugpak softie or surplus forces issued version. Don't expect either to be as warm as a decent down jacket. I recently replaced my Rab with a Berghans Saunda (I think) and its very good. Lots of pockets, down on both sides of the hand pockets, comfy fleece round the neck and a semi waterproof lining. It also has sythetic wrist sleeves with thumb holes, a great toutch.
If you want to bivvy out in the conditions we have at the moment you need a lot more than a buffalo shirt. I recently spent some time in Otterburn in Northumbria. It was -8c and it was buffalo and full softie suit the whole time for me when I wasn't moving.. but then I am a complete wuss in the cold.