> because presumably Americans don't get enough sunlight to produce sufficient amounts.

UHT denatures vitamin D, hence UHT and Pasteurized milk has lower vitamin D levels before the sterilization process.

I don't know any adults who drink milk. I'm sure some do, but I wouldn't count on that as the solution to Vitamin D deficiency.

White bread and processed grain products in the UK always have things like iron, calcium, vitamins, riboflavin added. I think it's to replace the loss of the nutrient-rich germ from milling.

Never known what riboflavin is, still don't today, but I saw it on the side of a cereal box every day as a child and the word is etched in my mind

Riboflavin is one of the vitamin B complex. I remember it because deficiencies are extremely rare, and there appears to be no practical upper limit on how much you can take, it is virtually impossible to overdose on. I mean, obviously, there is some limit, but it's sky high.

Which makes the supplementation of it a particular joke, in my opinion.

That it was discovered as a "vitamin" at all is probably more to do with it being easy to spot (very brightly colored) and easy to chemically handle and a few other accidents of history more than anything else: https://en.wikipedia.org/wiki/Riboflavin#History (I am editorializing that a bit and I freely admit that.) It certainly doesn't stick out like, say, iron, which has recognizable deficiency symptoms and plenty of people in the real world with them.