The most common understanding of lactase persistence frequencies is 'the fitness argument', that it is a super-trait that became widespread because of the direct benefits of producing gut lactase as an adult. This was always problematic and stupid, just look around the world. Plus, most non-LP people are not necessarily lactose intolerant.
But we've learned several things about the Corded Ware and Bell Beaker populations thus far 1) they tended to be darker skinned 2) had only begun the frequency increase of LP and 3) that those descendants went through a rapid selection process subsequent to the arrival at their new homelands. In other words, they were in a process of becoming whiter and more lactase persistent.
|Fig 5. Frequency of LCT (rs4988235) (Olalde et al, 2017)|
Now, if you have always lived in Northern Europe, or if you've never been to Northern Europe, one thing that is not so obvious is how cloudy, foggy and musty a place like Britain is. Vegetables are anemic throughout most of Europe. Even sunny days feel less radiant in the North.
Another thing that we continue to learn is the commonness of rickets. In fact, one of the Scottish Bronze Age children (I2569) had died, probably because of rickets. Rickets is returning again to Britain, and the polite British media has so far failed to report that all of those cases are among immigrant children, not British children.
To put another way, there is a combination of traits that work together that include skin color and lactase persistence. One way to test this hypothesis, is to test children like I2569 and see what derived alleles they have. On average, probably more of those rickets children did not have these alleles.