[whiddershins]

Yes, but vitamin D production relates to direct sunlight ... I think the angle of the sun and amount of atmosphere it has to go through affects UV amounts so a warmer European country does have less direct sun therefor more potential for vitamin D deficiency ...

TBH I am not super solid on every one of these details.

[spiralx]

That's a good point, but this paper seems to indicate that the amount of sunlight required over the summer in order to build enough vitamin D to last over winter isn't much for white Caucasians in the UK - nine minutes each day at lunchtime between March and September, assuming shorts and t-shirts are worn during the summer:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5946282/