As I'm quite often in both places, I've made a list of various things I miss from the UK and take over there.. and some other US things you can't easily get here to throw some light on the differences...
Things I have taken with me or bought from import stores in America because they don't seem to sell them in regular stores:
McVities Digestive Biscuits - not really a thing there. See also hob nobs.
Ribena - because blackcurrants aren't really a thing, and neither is diluting cordials.
Heinz Soup - the preferred choice in America is Campbells condensed soup which, ironically, needs diluting with water.
Jaffa Cakes - not a thing at all
Marmite - probably seen as a Brit peculiarity the world over
Branston Pickle - not a thing at all.
Heinz Beans - US equivalent is 'Bush's Beans' which are flavoured with meat and sweet tasting molasses (vom)
Bisto gravy granules - I've seen gravy sold in cartons/tins - I don't trust it.
Paxo stuffing - not a thing / only found in import stores
Curry sauces - eg. pataks, uncle bens etc. - Not enough South Asian influence to be a thing.
Gold blend coffee - replaced with Tasters Choice which is almost the same
Brit chocolates - of the ones I'm familiar with you can get KitKat, Twix, Snickers, Crunch, and Cadbury Dairy Milk (all with different quality/tasting chocolate), but no Mars Bars, Toffee Crisp, Bounty, Crunchie, Picnic, Boost, Turkish Delight, Maltesers, Aero, Minstrels, Revels, Star Bar, Double Deckers, Whispers, Milky Bar, Buttons, Fry's, or Yorkies
Things I wish I could take with me but can't cos perishable, too heavy, or meat:
Taramasalata - can't find it anywhere. Can't move for hummus, guacamole or salsa though.
Sausage rolls - just not a thing - see also pork pies, ginsters pasties, cornish pasties etc etc. nearest you'll get is an apple pastry pie
Waitrose microwave Indian meals. No Waitrose period #fwp
London Pride - you can find it sold as bottled beer, but it's a speciality import
Things I was mildly surprised were easy to find and sold all over:
Tetley Tea bags - labelled 'Tetley British Blend' no doubt aimed at Brits/Britophiles - black tea isn't as popular as green/fruity tea
Boddingtons ale (in cans) - called 'Boddingtons Pub Ale' there - presumably to make it sound more British
Newcastle Brown Ale - seems to be big all over the US, for some reason, which is good cos I like it.
Things from America I'd buy in the UK but can't find/get:
Trader Joes Chimichurri Rice
Enriched quick cook rice / Minute Rice
Mexican coke
Bud Lite
'Tide' washing detergent
Legal medical grade smoke
Massive size Listerine mouthwash
US style soft toilet paper
Fibre 1 cereal