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 winte

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