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?
Coffee. I used to be a coffee fiend, I drank up to 12-13 cups a day, and only stopped because it was worsening my anxiety. I live in a coffee producing country and learnt how to make a good cup in an espresso machine, even got all the doodads to make the process standardized and get the exact same cup every time.
I can only drink coffee made by select hands now. Everything else tastes like jet fuel, and it's worse when travelling.
The sad thing is that it is buyable but not in places below like 200k population. In Europe idk how far third wave type coffe got everywhere else but I'd imagine in international cities it's largely available all over the world.
It's frankly insane how different a specialty coffe place is from a Caffe. And how many specialty coffe places there are in big cities/ university towns.
The provinces seem to not be able to sustain good coffe unfortunately. Unless there is someone who doesn't care about the balance sheet.