Flea Beetles that had nothing else to eat ;-)....

Between the apparent spots there seems to be tiny specks that resemble spider mite damage, any webbing on the bottom of the leaf? Also, in your top pick I see what looks like a younger leaf in the lower right corner that has no spots so maybe whatever did the damage is gone now? It doesn't strike me as a plant disease.

Floral UK. Yes we did have a hail storm. It wasn’t long and pretty weak, but it did hail a week back. Would hail cause those markings?

Could also be Cercosporella brassicae. Otherwise known as Turnip White Spot. Pretty common in brassicas. Copper fungicides are alleged to work on it. If it gets worse, and you don't see any insects, that's probably what's going on.

Yes, hail wil make smal wounds which will turn whitish, as on that leaf.

I suggested hail because the marks look to me like physical scratches which have healed over. They're superficial and spread evenly over the leaf in a spatter pattern. We can't see the whole plant but are these marks on all leaves or just on ones which are fairly flat and near the top of the plant. If the lower leaves, which would have been protected, show no damage that would support my theory. In my experience flea beetles make more circular marks and they go through the leaf, not just scraping the surface. And I've never had them on thick brassica leaves. Some of the marks in the photo are just straight lines.

You are correct. Straight up leaves are marked much less, as are those which were more protected. Marks do not go all the way through, for the most part, and there are no webs to indicate spider mites. Thank you very much.

Sometimes the simplest explanation is the answer.