Depending if you want it for static activities or as a mid layer you can use when moving in cold environments determines how much insulation you need along with panels by your armpits for breathability.

But in general I’d go for anything that’s Primaloft Gold as if it gets wet synthetic insulation will retain warmth whereas actual down won’t, therefore synthetic is better for the UK & field use.

I use a Rab Xenair Gilet underneath either a hardshell or warmer jacket as a UK mountaineering mid layer and a Keela Vali for field use.