To be fair, packages have a hell of a lot further to travel to get across USA than here in England, so the longer shipping time would make sense.
As to your predictions, I agree with it being US-only (if it happens), but I would expect good quality. I mean, it's not like they're some random company with no understanding of the web, they do have AWS' infrastructure to use.
They do, but they aren't clones of each other. I'm in southeast Michigan and I often see Maumee OH and Hodgkins IL as the source, but I also sometimes manage to get things shipped from Nevada. Our diapers come from Lewisberry PA, I've got some nuts from Philidelphia, and we got a book a couple months back from Louisville KY. I certainly do not simply get everything from my closest distribution center.
I've no idea about their US distribution, but presumably in a country so much bigger than the UK, if they did have a massively larger distribution network, they would also have to charge more for the service. Maybe they opted for a slightly less impressive service rather than a higher price tag.
As to your predictions, I agree with it being US-only (if it happens), but I would expect good quality. I mean, it's not like they're some random company with no understanding of the web, they do have AWS' infrastructure to use.