Also as a British expat, Sainsbury’s and Waitrose in Bath had all of the types of onion you’d find in the US pre-pandemic - though I haven’t been back since. I’d imagine Brexit may have ruined that.
I haven’t been back in a few years, but the selection of onions in my local Kroger would be staggeringly exotic next to the offerings of Tescos of the north-east.
Yes, I suppose it doesn't really surprise me that the regional variation would be different. Bath didn't have a Tesco until recently, but it did have a Safeway/Morrisons which had a slightly worse selection than the "posher" two. It also had a farmers market though which was by far the best place to buy anything.
I am absolutely baffled by the local grocery store’s (Midwest US) supply of potatoes. They have something like five+ varieties of midsize ones, two types of big ones, bags of both, and an entire end cap dedicated to various little bags of tiny potatoes.