If you went into an English tea room for afternoon tea and they served you that monstrosity and claimed it was a cucumber sandwich, you’d be perfectly within your rights to burn the entire place to the ground and not a single judge in the country would convict you.
If the thread had been titled “how do I make a bahn mi” then I’d have upvoted you to the moon. But it wasn’t. It was asking how to make that most English (not British, but English) of foods, a cucumber sandwich. And in case that wasn’t a clue enough, the community is Ask UK. Not Ask Vietnam or Ask America.