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?
Here's something that you can't buy outside of Italy: mozzarella. I tasted proper mozzarella in Tuscany and it's nothing like the shit labeled mozzarella sold in supermarkets around the world, and for a good reason: real mozzarella has a shelf life shorter than Trump's attention span.
There are tons of places that have fresh mozz
We have a deli here that makes fresh moz daily, you can find places that do it all over. Shelf-life really only keeps it out of supermarkets. The problem for many forms of cheese in many countries, and especially the US, is the requirements around pasturization. Completely changes the texture and taste. And for moz specifically, the lack of Buffalo.
Buffalo...sauce? Buffalo, New York? Buffalo the ungulate? I am confused
Buffalo the animal (I think it's water buffalo for mozzarella)
That's called mot-zer-eller, if I remember right.
no no, it's "Moe's a fella"
You mean those watery packets of cheese I sometimes buy aren't supposed to taste like watered down kangaroo testicles?
Look at the plus side: at least you know what kangaroo testicles taste like.