Matalan also sells Heat Holders as do most outdoor markets. My wife wears them all through the winter. Personally, i don't feel the cold in my fingers or Toes. So i just wear a thin pair of Cotton socks whilst wearing my boots. Havent owned a pair of gloves since my Mum made me a pair of mittens with string going up my arms towards the other one. Just thought that was over 50 years back.
I have a variety of thermal socks ranging from sealskins to knee length golf jobbies off the bay! I also have some socks made from neoprene ! We are all different but I find that my feet get cold if I am just sat there in icy conditions! Circulation is the key so any would be OK if you are walking about! I wear warm neoprene wellies too! If I am walking any distance my feet glow with heat!
Never bother with any particular socks. Skee-tex field boots cover most weathers and if you're pretty static Skee-tex moon boots are guaranteed to -50c. No issues with cold feet which is just as well as it's a bit tricky putting them in your pockets to warm 'em up..
I just bought a pair of thermal socks from Primark, extremely warm and comfy. Only four quid so don’t expect them to last for too many washes but that’s how Primark operates, and very successfully too.