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?
Stuffing and mashed potatoes.
Most places have their stuffing way to moist. I want stuffing, not bread that looks like it was dropped in water. Boxed stuffing shouldn't even be sold. It tastes like garbage.
Get some bread. Tear it up. Let it dry. Add some chicken broth. Add some seasoning. That means go to store and buy the different seasonings. Like garlic powder. Sage, thyme, etc.
Then put it on the oven. The moisture comes from gravy.
Mashed potatoes... Yeah most times people add way to much to the mashed potatoes.
Edit and for the gravy that means you make a chicken or a turkey you get the broth and you make the gravy.
in many places the stuffing is moist hecause itbus cooked in the actual bird