The trouble with bringing British tea is that it won’t be formulated for the local American water. Anyway, it is easy enough to buy many types of tea in America. They just don’t know how to make it properly.
Don’t bring cheese, but you might go back with some.
Being Wisconsin we would like some cheese.
Maybe not what most people would crave, but when my family visits the UK, they always bring me flying saucers: little saucer-shaped packets made of stiff, edible rice paper filled with fizzy sherbet. For some reason, fizzy sherbet is completely unknown in the US. Pop rocks we have, but no fizzy sherbet.
Flying saucers are a cheap “old-fashioned” candy, so it’s a bit like asking someone in the States for Necco Wafers, but they’re my choice.
After that, Aero bars and Cadbury Flakes. You can get many Cadbury products in the US under license from Hershey’s, but not Flakes. Aeros and Flakes both seemed like products that would be no-brainers for American companies, since they’re basically ways of replacing large volumes of your chocolate with air and selling it for the same price as a solid bar. I always thought maybe they believed American consumers were too cynical for such things (though it seemed unlikely to me), but njtt makes me think maybe climate is a more significant factor.
Most of the things mentioned in this thread are readily available at any indian grocery store.
I haven’t tried them, but my fiance was raised on british treats in india, and loves them.
Interestingly enough, the local gelateria has Horlick’s flavoured gelato…
Some stuff with the Harodds logo. Don’t they sell tea there with their logo on it in those cool little tins? I’m not a tea drinker but I have a couple of those tins and I like having them around for my daughters to stash stuff into and play with.
Almost like the custom made ceramic crockery that KLM Airlines used to give out to their first class travelers, but maybe with not so much cachè.
Slade records, dammit.
No one over hear appreciates the awesomeness of Noddy Holder.
OK not UK per se, but my missus misses these Irish treats:
Four Star Pizza
Club Orange (like Fanta but a lot nicer)
KP Peanuts (they have them in the UK too)
Batchelor Baked Beans (American beans are different somehow)
Irn-Bru ('tis made from girders)
Angel Delight
Big one: JAffa Cakes. Take lots of them. And Brown Sause, not Daddies.
Since when? I’ve lived in South Carolina almost all my life and I assure you we do not keep the chocolate in the fridge. (We don’t leave it in the car, either, of course.)
Sure
I have a box you can climb in, it will let you travel through time at a 1 - 1 ratio. If you want to pay for UHaul, you can travel in space too. I’m a nice guy, so I will paint it blue. However, an attractive female companion will cost you!
A lot of those things are fairly readily available. Marmite, for instance, I know I can find at Meijer, Treasure Island, and Cost Plus World Market, off the top of my head, and I suspect Trader Joe’s and/or Whole Foods would also stock it. Same with Golden Syrup and McVitties products (though not all of them.) Golden Syrup, especially, is used quite a bit in Louisiana, although it’s not quite as well known here in the Midwest.
I love Hob Nobs.
But I’m too cheap to pay six or seven bucks for a tube of them, so I buy Voortmans. Not as good, but available and cheap.
Candy without gelatin!
Can I have some Skittles, please? 
On my last visit to the UK I returned with Mark’s & Spark’s bagged toffee candies. They’re individually wrapped & usually found in the food court by the checkout. They went over a treat!
Whatever you do, DON’T bring marmite. Spawn of satan, that is.
Amy Pond.
Okay, fine, you’ve convinced me - I’m going to try marmite. I like strong, strange-tasting things, so I figure I’m 50/50 on liking it.
[Ackbar] It’s a trap! [/Ackbar]
I am not saying the things on my list cannot be found for sale anywhere in America. If that were the game, then there would probably be nothing at all to list in this thread. The OP does not ask what UK goods are absolutely unobtainable in America, it asks which are rare. None of the things I listed are standard American fare, routinely available in most ordinary supermarkets, and most Americans are probably not familiar with any of them.
Incidentally, I have frequently shopped at Trader Joe’s and occasionally at Whole Foods, and never saw Marmite (which I see my U.S. English spell checker does not recognize as a word) in either, and, believe me, I looked! I am not familiar with Meijer or Treasure Island (I do not think they had any stores anywhere near where I lived), so I don’t know. Yes, Cost Plus World Market often sells it (and, on occasion, several of the other things on my list), but note the name: World Market. They are, specifically, a shop that specializes in goods imported from other countries, and not readily available in America, and you will be paying the sort of high prices relevant to goods specially imported at low volume to satisfy a very small market (mostly expatriate Brits). Indeed, that will be true anywhere you buy Marmite in America.
In fact, I could name you about three other places in the western San Gabriel valley where you can fairly reliably get Marmite (probably the easiest thing on my list to find), and I have even seen it (on very rare occasions) turn up briefly in one of the major chain So Cal supermarkets, such as Ralphs or Pavillions. But the point is not that it is non-existent, but that it is rare, and if you want to have it (as I did) you need to make a special effort. The same applies a fortiori to all the other things I listed. (And if Marmite is relatively common compared to, say, Rowntrees Fruit Gums, it is not because many Americans want it, but because many expatriate Brits, such as me, cannot live without it!)
Perhaps you are right that Golden Syrup is routinely available in Louisiana. I have never visited the state, so I can’t contradict you. I believe I have only once, in 20 years, seen it in Southern California (or other states where I have relatively briefly lived) outside of a specialty import store. However, Louisiana notoriously has a culture very distinct from that of all the other states, and it is certainly a long way (culturally and geographically) from Green Bay. I think it is a safe bet that the OP’s colleagues there will not be familiar with it (unless, perhaps, via contact with Canada).
Well, I guess I learned to do it from my wife, who has lived in Southern California all her life. Maybe not everybody does it, but, unless you are one of those people who keeps the air conditioning on high all summer, effectively turning your whole home into a refrigerator, it is a practice I would strongly recommend. Even chocolate made for the California market (I think it is deliberately made to have a higher melting point than British chocolate does) turns soft in the So Cal summer heat. I think you will find it is much nicer if you keep it in the fridge. I don’t think chocolate needs to be cold, but cold does not harm it, and I do not think I am alone in preferring it to be crisp rather than soft and squishy.