From NHS Scotland:

"In Scotland, we only get enough of the right kind of sunlight for our bodies to make vitamin D between April and September, mostly between 11am and 3pm."

https://www.nhsinform.scot/healthy-living/food-and-nutrition...

Personally I found that taking Vitamin D supplements made quite a bit of difference - and I spend a fair amount of time outside (~3 hours each day).

And in Norway we often don't see the sun during certain months, due to it only being up for a few hours in the middle of the day (when we're working). And even if I was outside I would be covered in clothes.

We have a saying here to take cod liver oil all months ending with R (in Norwegian that's September to Februar) to get both omega 3 and the vitamin D.