In Coventry and around Liverpool the roll is a batch, and the word carries a clue about how it bakes. A batch is a soft white roll proved and baked crowded together in the tray rather than spaced apart, so its sides come out pale, tender and tear-away soft where the rolls touched, with only the top taking any colour. The result is a roll that is mostly give: a thin soft skin over a close, slightly chewy crumb, with no real crust to resist anything. It pulls apart in the hand instead of cracking. That softness is deliberate, and it is the entire reason the batch works under bacon. The bacon is the thing every region agrees on; the batch is what makes this the Midlands and Merseyside reading of it.
What the batch does with hot bacon is yield and soak in the right proportion. Rashers fried until the fat has run and the edges have caught come off the pan carrying grease and salt that has to be absorbed without leaking; the tender crumb takes a controlled amount into its base and goes dense and rich there while the lightly coloured top stays dry. The build asks for little beyond timing: butter the batch while it is still cool so the fat does not run straight through it, lay the bacon in hot so the heat keeps working in the bread, keep sauce on the inside so the outside stays clean to hold, then close it and let the soft crumb fold around the filling rather than perch over it. Eaten before the steam has fully slackened the roll, it gives a dry, faintly springy top and a base gone heavy with bacon fat, the rashers chewy and salt between.
A short distance away the same sandwich answers to other words. It is a bap across much of England, a barm in Lancashire, a cob through the East Midlands, a morning roll in Scotland, a stottie in the Northeast. Drop the bread word and it is a bacon butty, or a bacon sarnie when nobody is being precise. The fillings branch the same way the names do: a fried egg with a yolk to be kept under control, a sausage with or instead of the bacon, brown sauce against red as a fixed domestic argument. Those deserve their own articles rather than being crowded in here.