I get the feeling you are being persuaded away from what you originally stated. Sleeping bag systems can be very flexible if you are prepared to be clever about things.
Yes, down is warmest by weight and - as an alpinist - I know it is fantastic for cold dry conditions. The alpkit bags are really good value and perform relatively well - but they are bulky and, even if used in goretex bivvy bags, they need to be aired or your body moisture will keep them damp. In summary, down bags are superb in the right conditions.
Hollow-fill synthetics, like Primaloft, are not as efficient - but we're in the UK, not the Himalayas! How about using a lightweight, 2-season, hollow-fill bag, and, when it's colder, use combination of: goretex bivvy bag, thermal liner (eg, meraklon), and - wait for it - wear more clothes in the bag! I have used a 2-season bag with the aforementioned liners/bivvy bags, etc, layers of thin thermals, then a duvet jacket (either wear it or put it over the top of you). This will keep you warm even in conditions close to zero degrees Celsius. And, the best thing is, in cold conditions you'll be carrying warm clothes anyway - so no 'extra' stuff required.
There are certain things that will also make you feel much warmer - insulated socks (down/synthetic) keep your feet toasty, which makes the rest of you feel warm. Try and plug the gap around your shoulders to keep warm air in - use the bag baffle and draw-cord, or some spare clothes. Finally, don't breathe into the bag! The moisture in your breath will make the bag damp - leave a small hole to breath through or - even better - wear an insulated balaclava.
Furthermore, ensure you are well insulated underneath - the weight of your body crushes the bag filling, reducing insulation, so you will ose a lot of heat through your lower surface in contact with the ground. An inflatable mat (Thermarest, Alpkit Airic, etc), or closed-cell foam mat will provide that extra isolation you need.
Buffalo sleeping systems are good - like the clothing though (and I should point out that I love my Mountain Shirt!), they are heavy and bulky.
Snugpak/ME/North Face synthetic bags are outstanding but ensure that you store them out of the stuff sack - let them loft, or the fill gradually loses its insulating properties.
Army bags are good too, but too hot for summer and they are bulky and heavy.
At the end of the day, it's horses for courses - and you won't get one bag that does it all. Decide where you want to compromise and then make your choice. You may end up buying two bags - one for summer and one for winter. You may find that a poncho-liner or lightweight duvet blanket will be fine for summer; then you can spend more on a winter bag.
Sorry to be getting a bit preachy - forgive me - but I got the feeling things were getting a lot more complex than they actually should be...