My guess is that any type of dish probably going to be something regional. That is, I think that it's going to be hard to find something that is widespread in the US but nonexistent outside the US -- someone will have gone and spread it, if all it takes is "get ingredients, mix". And for something like that, you can do it yourself, if you're willing to do the cooking.

There might be some sort of processed/manufactured foods that aren't sold outside the US -- like, candy bars that have tailored regional variations -- but the UK tends to have some sort of reasonably close analog, is also a leader in that sort of stuff, and I suspect that it's typically possible to get that abroad from someone exporting them in small amounts if you really want a US-specific variant.

Maybe Dippin' Dots. They're flash-frozen ice cream done with liquid nitrogen, and you can't really readily ship them without a fancy logistics chain. WP says that they do operate in 14 countries, so it's not just the US anymore, but I don't see them in the UK. It looks like, on Google Maps, Chicago does have some Dippin' Dots stores that sell them. kagis It looks like there are some caterers and restaurants in the UK that will make some sort of liquid nitrogen flash-frozen ice cream, but to my quick glance, it looks kind of different.

There are some plants that don't ship well that you can sometimes find in a region, so you can't really get it outside of that region. I remember some discussion about things that people who moved to the US couldn't find there, and someone from Australia mentioned some kind of Australian fruit. kagis I don't think that it was the quandong, but same idea. Probably have to be in the right part of the US and in the right season for something like that.

The huckleberry refers to several different berries, some quite different, but Vaccinium membranaceum is my favorite, grows around Idaho. Attempts to domesticate it have not met with success, so you either pick it wild or get it from people who pick it wild. I don't think that you could get that outside of the US or Canada other than in some sort of preserved form -- I can't even get it in California (though California has its own huckleberries, though not, in my opinion, as appealing).

The pawpaw doesn't look like it's really grown in Europe.

https://en.wikipedia.org/wiki/Asimina_triloba

The genus Asimina exists on only one continent, North America.

The problem is that given that this is winter, it's probably not the ideal time for fresh forms of any fruit that doesn't ship well.

I'm American but I think deep dish pizza sucks

Italian Beef Sandwich

Not sure about the UK but I've had pizza in both Italy and Germany and they are NOT like American pizza.

Red N. 3.

Chicago? I dunno. I know what I'd suggest by NYC - a good pizza, a Rutt's ripper, and a proper Philly cheese steak - with da wiz. Out there, I guess chicken & waffles or maybe biscuits & gravy? It's the wrong time of year for green bean casserole.

Shared between us and them, knishes, kasha varnikes, and the Reuben sandwich isn't unknown. So find a delicatessen.

Otherwise, chili. Homemade.

That taco bell shit. All of it

Idk