the first question is have you been to a doctor about this? There are various deficiencies that can lead to poor circulation/feeling cold - thyroid I think? Vitamin D maybe? I can't remember off the top of my head, but it's definitely a thing and for you to feel this below 20 means you're really very very sensitive to cold.
I have spent time in countries that are, arguably, colder. Weirdly, I didn't find it as bad. I can't quite work it out. is it to do with the damp here? the way our houses are heated or not heated? I'm not sure, but I suspect it's relevant.
I do think it's easier to stay warm than to warm up when you are already cold. So things like warm bath/shower, in a warm/heated bathroom, with a heated towel and then immediately getting dressed before you leave that room. Ditto electric blankets to pre warm the bed before you get in.
Thermal layers - I loved a good thermal vest and leggings - they need to be proper thermal, and well fitted/snug. And never too small - NOTHING made me colder quicker than that tiny gap between the bottom of my thermal vest and the top of my thermal leggings.... (in the days before peri struck and my overall body temperature increased masively!)
Warm feet, hands, neck and ears - cashmere socks and gloves (and I have been known to wear thin cashmere gloves then some kind of man's over sized ridiculous therman glove over, with wrist warmers), hats and scarves. When especially cold, I used to wear thin silk scarves (proper ones - not the satin numbers that are technically silk but seem to have weird properties) with a warmer and larger cashmere scarf over the top.
My mother swore by heated hand warmers in the pockets of her coats. I like to leave socks and gloves on radiators before I put them on!