Expedition sleeping Bags designed for temperatures below -10ºC. Generally these will be too warm for UK use, though some may be applicable for Scotland in winter.
A good quality down sleeping bag should have a lifespan twice that of an artificial fibre bag and be warmer and pack down smaller for the same level of warmth. However they are more expensive and care needs to be taken that you do not get the down wet. Artificial fibre sleeping bags are usually much cheaper for the level of warmth than down ones, stay reasonably warm if damp and can be easily dried out if they get wet. However they are heavier and bulkier for the level of warmth they provide, and will not last as long as a down one as the fibres break down with repeated compression.