Some "spert" says it is toasted white bread with slightly salted butter and back bacon.
Well, that's not my perfect bacon butty. Mine is untoasted white bread with any sort of butter I find in the fridge but only for one slice, the other slice soaks up the offerings from the bacon, and that has to be middle cut bacon just going crisp.
I bet that every bacon butty eater has theirs as perfect without any input from some self-professed "expert".