In Lancashire the roll is a barm cake, and saying it any other way marks you as not from there. A barm is a soft, flattish white roll with an open, springy crumb and a pale floured top, the word itself pointing at the leaven that lifts it. It sits lower and looser than a tall bap, with a crumb full of small irregular holes that compress flat under a hand and then spring part of the way back. It is not a crusty roll and it is not meant to be one. The barm is engineered to be squashed, and a barm that fights back has been baked wrong. The bacon is the part everyone shares; the barm is what makes this Lancashire's version of it.
The open crumb is what the barm contributes once hot bacon goes in. Fat rendered out of frying back bacon, with the rind crisped and the edges browned, soaks into those open cells and weights the lower half of the roll into something dense and savoury while the floured top stays dry and matt. The build is unfussy and the timing is the only real discipline: butter the barm while it is still cool so the fat does not pass straight through, lay the bacon in straight from the pan so the heat is still working, keep any sauce on the inside so it does not run down the outside, then close it and press gently so the loose crumb collapses around the filling rather than sitting above it. Eaten quickly the contrast holds: a soft chalky lid, a base gone rich and translucent with bacon fat, the rashers giving salt and chew between.
Move out of Lancashire and the identical sandwich changes its name with the county. It is a bap across much of England, a cob through the East Midlands, a batch around Liverpool and Coventry, a morning roll in Scotland, a stottie in the Northeast. Set the bread word aside and it is simply a bacon butty, or a bacon sarnie in passing. The fillings vary the same way the bread words do: a fried egg whose yolk has to be managed, sausage in or alongside the bacon, the brown-against-red sauce question that splits households. Those deserve their own articles rather than being crowded in here.