Alpkit bags are worth a look and get a lot of positive comments on here. I have the Mountain Ghost bag (as advertised at the top of this forum), it's synthetic but feels very similar to down and packs down very small. Everyone will differ in their opinions but I find the comfort rating (-0.8 c) to be about right as I've used it through the winter in temperatures that have floated around this limit. My camping buddy has one of the Alpkit down bags which also seems to perform to it's stated comfort limit.
Personally, if I was you I'd probably focus on getting a good quality 3 season bag with a comfort limit that'll take you down to zero or thereabouts. That'll keep you warm in all but the coldest of conditions that the UK typically experiences. No need to get a huge sleeping bag that keeps you warm down to -1squillion if you're not likely to camp out in those arctic climates. If the forecast tells you it will get colder then get yourself a nice wool blanket. In any case the blanket will prove to be a good versatile bit of kit. Alternatively, invest good money in the bag that you'll use all year round and then spend 30-40 quid on a British army arctic bag that you can use when the temperature plummets. I don't own an arctic bag but I know lots of people that do and not one has ever complained of having a cold night in one!
Personally, if I was you I'd probably focus on getting a good quality 3 season bag with a comfort limit that'll take you down to zero or thereabouts. That'll keep you warm in all but the coldest of conditions that the UK typically experiences. No need to get a huge sleeping bag that keeps you warm down to -1squillion if you're not likely to camp out in those arctic climates. If the forecast tells you it will get colder then get yourself a nice wool blanket. In any case the blanket will prove to be a good versatile bit of kit. Alternatively, invest good money in the bag that you'll use all year round and then spend 30-40 quid on a British army arctic bag that you can use when the temperature plummets. I don't own an arctic bag but I know lots of people that do and not one has ever complained of having a cold night in one!