Fashion

Something like this for extreme cold ?
They have Helly, Columbia, Haglofs, Rab, Berghaus and Fjern on sale as well most of it half price or less. I bought a mountain hardware puffa from them last year almost half price and its been so good its made all my other non waterproof winter jackets redundant.
https://www.sportpursuit.com/catalog/product/view/id/2202063

something like -20C

For a fashion item in the UK? Why?

Alpkit or used (Rab/etc.) is probably most budget

If you want to go super budget I’d get an ex army parka and line it with a cheap but decent lightweight down jacket

Weirdly its kind of a unique jacket in today's marketplace - in the sense that it is an expedition parka but with big down filled baffles, which isn't that common nowadays and very much is a throwback to old school down coats of the 40s and 50s.

I'd just search for 'expedition parkas' or 'everest parkas' on eBay or whatever your local equivalent is and try to find one that you like - if you really want something in a similar style to the Fjallraven without spending Fjallraven money, that is.

I'm expecting something like -20C

I've worked for extended periods in -20ºC, always toasty wearing my Rab Expedition parka, handmade in Sheffield back in the day, they cost about £150 on ebay.

for your legs, thick merino leggings, plus Mammut Champ pants (double-layer softshell Schoeller fabric), have worked well. alternative pile-lined trousers by Rab are available.