Ummmm... Humans are warm-blooded too and we still rely on clothes to keep us warm doing the winter. The exact same thing applies to animals. So if they don't have fur or anything to keep them from the cold, no they would not be fine at all.
Big ones likely would be fine for the most part, they've got a lower surface area to volume ratio, really would mainly be the smaller ones.
The Brittish Isles? That's way too cold. And it feels like your just asking for species and don't care about the DLC aspect.
England & Ireland aren't really consistently cold (
particularly in the south where it's warmed by Caribbean currents), so just having heaters and a heating cost during colder months isn't the most extreme thing, assuming they wanted to put seasonality in a map at all (
they could also just make it a constant cost, without the map changing at all).
Hypothermia could be a new disease and general diseases could also be more prevalent. You could also have shelters if you wanted, there's ways to solve the problem/present a problem to be solved by the player, even if it's as simple as heater coverage.
