To care that my children’s great grandparents were first cousins?

[Antipoodean]

Not particularly unusual perhaps, but I come from a very diverse gene pool myself (different hemispheres). I only learnt that my MIL’s parents were first cousins after we had children. Would this bother you? My dcs are healthy and I am not worried about them, I’d have had them
with the knowledge, it’s just a bit disconcerting.

No, I’m not claiming that. I’m not claiming anything. I’m just saying you can’t use that data as meaningful for the whole U.K. that’s it.

Fox R. Kinship and Marriage: An Anthropological Perspective. Cambridge: Cambridge University Press; 1983.

By the mid-20th century, less than 1% of marriages were with cousins [in the UK]

1 set of great-grandparents being first cousins isn't anything to be concerned about in terms of your children's health.

It is a little bit ick by modern standards, but 60-100 years ago, it wasn't very unusual, especially in small communities where the dating pool wasn't very large.