For example, Marmite Crumpets don’t exist. You cannot buy them at the supermarket. To be clear: you can buy crumpets, you can buy marmite, you can buy butter; but you have to assemble them at home.
If you walk into a breakfast cafe, they will happily serve you sausage / egg / bacon / french toast / bubble / squeak (whatever that is). But no marmite crumpets. If you ask them to make it, they will give you a very strange look. It’s not typically offered. It’s something you just have to make at home.
It is unbuyable. Any tourist who comes to the UK to try a Marmite crumpet would need to bring a toaster or an oven with them, or quickly befriend a brit and hope that they have all the ingredients at home.
It’s not a secret. You just can’t have it.
*munches into crumpet thoughtfully, and salivates at the juicy savory delight, whilst staring at you pityingly and condescendingly*
Anyway, what’s something that I could never experience unless I made it myself in your local?
The Cannibal Sandwich, which doesn’t actually use human flesh, but is also not a sandwich. Anyway, you take a slice of rye cocktail bread, spread on some raw, ground beef, then top it with some sliced onion, salt, and pepper. You can’t get it ready-made, because nobody likes e. coli or salmonella poisoning. In fact, you have to make special arrangements to get the beef ground by a butcher in a clean grinder, and pretty much eat it the same day.