Very interesting...I've always keptI always keep a NATO folding shovel, 2 blankets (1 surplus 60/40 wool mix & 1 pure wool from the Salvation Army shop), wellington boots + socks, LED headlight + spare batts and a good stock of hand tools in my car all year round. I also have standard supplies and some extras such as some break/clutch fluid(from personal experience), plenty of rags, spare bulbs, Holt's tyre weld, extra Halfords ready mix screen wash which freezes at -9 and old car mats (very good when inspecting underneath the car, etc).
When the temperature goes way down, I'll add a hexi burner + brew kit, thermal insoles, charcoal hand warmers, matches in a sealed case, a 24h ration pack and 3l of water. I'm looking at getting some SealSkinz gloves this year too. I spent a week cutting slate tiles on a wet cutter outside in the snow a long time back and my fingers blew up like sausages. It happened again with my toes when my DeWalt work boots got soaked walking to site. Turns out I got chillblaines. Very sore, very uncomfortable. Lesson learnt.
Watch out with warning triangles, I'm not entirely sure, but I don't think you're allowed to use them on motorways.