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?
I feel like that's something only few people could actually make
True, but it's an important reason for many people to start to learn how to make their own clothes. It takes effort, but one can learn how to do this. And it used to be a very common skill. With today's junk on the market, we have a good enough reason to start learning.
I can sew, but finding decent fabrics is hard. Back in the 70s I made all my own clothes and I can still remember some of the fantastic fabrics I used: a ming blue paisley sateen cotton; a red denim (for a duffel jacket with a toning floral for the hood lining); a soft purple lightweight wool; a dark green raw silk; glorious Chinese rayon florals in rich colours. So much choice!
I patch my old socks with older socks, if that helps. And I fix armpit tears with a rough stitch. That's about it.