The "big cheap" sleeping bag alternative is Aegismax; there are several models, for example this larger warmer one:
https://smile.amazon.co.uk/AEGISMAX-Urltra-Light-Sleeping-Urltra-compactable-Regular/dp/B01CD21YA2/Same temp ratings, sightly less weight, slightly higher fill power (Goose vs Duck). The Aegis will NOT be ethical and the Rab WILL be made better. The Aegismax is a clean £115 cheaper.
When it's cold I use a 100% silk sleeping bag liner, I'm a great fan of JAG BAG from an Edinburgh company who use Kiwi workers.
https://www.terrevistatrails.com/Another tip when it's cold is to wear a baselayer, e.g. Merino Wool long-johns & long-sleeve t-shirt.
A warm sleeping pad doesn't hurt either, my matress of choice is Klymit Insulated Static V Lite. R-Value 4.4, half a kilo, and under £100. Thermarest XTherm is even warmer, R5.7 for £150.
https://smile.amazon.co.uk/Klymit-Insulated-Camping-Sleeping-Flip-Valve/dp/B07TKD1MKB/I use a (good) drybag for my sleeping bag, it's always the last thing to get unpacked. I keep a "wet" drybag full of wet socks & grots etc., but my tent came in a waterproof stuff bag so I use that. Otherwise it would get it's own drybag.
A huge bin-bag to keep your bobbins dry inside your pack is fine (having one big pack, be it drybag or rubble bag, is more space-efficient than lots of little dry bags). I use one. But I also use a SnugPak cover. My pack is "shower proof" but I found having the cover on actually worked really well, it keeps 90% of the storm out (the bag itself, finally the liner, does the rest) and it helps protect the lightweight, easily-ruined material. Otherwise I'd be ******* bricks at every walkers fence. I've also found the cover to be useful when kneeling in the wet, putting stuff on otherwise wet ground, etc., so it can have utility. Even so, it's only A line of defence not a be-all-end-all.