My Doctor told me. Also see the NHS page https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-....

You can see the development of the UK guidance.

There's a consensus statement from 2010, here: https://www.nhs.uk/Livewell/Summerhealth/Documents/Concensus...

And there's discussion of the new guidelines (with links to the newer research) here: https://www.nhs.uk/news/food-and-diet/the-new-guidelines-on-...

This is simply a result of how vitamin D is manufactured, and the lack of sunlight in northern latitudes.