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?
Some cafes will do it - not as standard, but a few - maybe try the ones trying to be 1-up from a greasy. https://seahousescafe.co.uk/the-breakfast-menu
As will many hotel breakfasts, there's often little single serving marmite things in with the single serving jam packets. I'd say about half the hotels i've stayed in with decent cooked breakfast have had it on offer.
I've also seen it in little roadside food van / trailer type things too.
Anyway, you want sainsbury's yeast extract instead of marmite, it's way gloopier and nicer tasting.
It just sounds wrong but I'll be on the lookout