I used one of these for 8 years in the Army. They were fine for any weather I experienced in the UK and W.Europe. You have the smaller version which my Battalion were issued with. I never figured out why as they were too small for the job we were doing. I must be honest I never used that one as I had the larger original version shown in your third pic. If you look closely you'll notice that soldier's Bergen is larger than yours. It's about 120 litre if I remember right. The Paras were issued them as standard back then. The frames and straps are the same on both size Bergen's and the frame was used to carry the Clansman radios as well. The larger Bergen will fit right onto that frame.
I carried between 120 -140 pounds in that Bergen for years and it stood up to everything and did it well, although mines was pretty much past it's best when I got out. The waterproofing had perished ( I always bagged up my gear anyway ) and the plastic coating was peeling from the frame. To be fair though it had been well used/abused. The back support straps inside the frame can be tensioned to suit and fitting another few is good, if you can find them. A closed foam sleeping mat can be folded up and stuffed in there for extra comfort although I never bothered.
Some will say these Bergen's are rubbish. I would say they did their job well. They are certainly robust. I'll be honest and say this though. We had officers who wouldn't allow us to use civvy kit so we didn't have a choice. Would I have bought something better had I had that choice? Probably. There wasn't much suitable alternatives around at that time although I know Cyclops Bergen were popular among the Royal Marines so I would probably have tried one.
Your Bergen and frame will be fine in UK winters mate.
Edit... Just realised you're in Alaska mate. Probably the coldest I experienced using that frame was in Bavaria. An unexpectedly cold few days reached around -20c if I remember right. The Bergen's probably coped better than we did.
