As this is a US centric borad you will probably get lots of recomendations for brands that aren't available in the UK
As you have already seen down bags vary enourmously in price but it is very unlikely that you need a top price bag (if you did you wouldn't need to ask on here about them). The price reflects the quality of the down, the material, and the build quality. The first thing to do is decide what you really need, ie temperature range, how important is weight, will you always be using it in a tent? etc.
Alpkit have a good reputation for bags that good but relatively cheap but they are often out of stock. At the other end of the scale PHD offer the very best quality and will custom make bags and other kit but the price reflects the quality. Rab and mountain equipment and many other makes are also good.
If you give some more details about what you want to use the bag for and how much you want to spend I might be able to give you some more pointers.
ps, many sleeping bag specs will give a 'fill rating' for the down used. This is a measure of how much the down expands for a given weight. The US and european methods for measuring this are different and the US method will give a higher figure for the same down compared to the european.