The butty is the national name for the thing, and the name points at what holds it together: the butter. A butty is bacon on soft white bread or a soft roll, and the butter spread to the edges is not a garnish or an afterthought, it is the structural and flavour bridge that earns the word. Cold butter laid on soft bread does two jobs at once. It puts a thin fat barrier between the bread and the hot grease so the base does not turn straight to paste, and it carries the salt of back bacon across into the bland sweetness of white bread so the two halves meet in the middle instead of sitting apart. Take the butter out and you have bacon between bread. Leave it in and you have a butty. The bread word changes across the country; the butter and the bacon are what stay fixed.
As a sandwich it works because every element is doing one clear job. Back bacon is fried until its fat has rendered and the edges have caught, which is what gives the salt, the chew, and the rendered fat the bread is built to absorb. The soft white bread or roll is chosen to compress around the rashers rather than fight them, soaking a measured amount of fat into its lower face and going dense and rich there while staying intact long enough to reach a hand. The sauce, brown or red, goes on the inside so it does not run down the outside, and it is there as the acid-and-sweet counter that stops a salty, fatty middle reading as one flat note. The timing is the only craft that matters: bacon in hot, roll closed and pressed gently, eaten before the steam has fully softened the bread, so a dry top still meets a fat-soaked base.
The same sandwich answers to a regional word almost everywhere it is made. It is a bap across much of England, a barm in Lancashire, a cob through the East Midlands, a batch around Liverpool and Coventry, a morning roll in Scotland, a stottie in the Northeast, a butterie in Aberdeen. Say it casually and it becomes a bacon sarnie. The fillings branch the same way the bread words do: a fried egg with a yolk to be managed, a sausage in or alongside the bacon, the brown-against-red question that settles differently in every kitchen. Those deserve their own articles rather than being crowded in here.