In Lancashire and across the Northwest the battered-fish sandwich is a fish barm, and the bread word is the thing being claimed. A barm is a soft, flattish white roll with an open, springy crumb and a pale floured top, the name pointing at the leaven that lifts it. It sits lower and looser than a tall bap, full of small irregular holes that flatten under a hand and spring part of the way back. The barm is engineered to be squashed, and one that resists the press has been baked wrong. That is exactly the behaviour a slab of chip-shop fish wants over it. The battered fish is the constant here, a fried fillet in a hard blistered shell, and the barm is what makes this the Northwest's reading of it rather than a generic fish butty.
The craft is the same coating-and-grease discipline the format always demands, met by an open crumb that takes it differently. The fish comes from the fryer and is rested only briefly, long enough that the steam under its shell does not soften the crust from within before it reaches the roll. The barm is buttered to the edges while still cool so the fat film forms a barrier inside the soft floured crumb rather than letting grease pass straight through, and the open lower crumb absorbs a measured amount of fried fat and steam and weights into something dense and savoury while the lid stays matt and dry. The fillet is broken to the width of the roll and laid flat so the barm closes cleanly, then pressed firmly so its loose, holey crumb collapses around the fish and binds the whole thing for a few bites. Salt and malt vinegar go on before closing so they stay inside.
Out of Lancashire the identical sandwich changes its word with the map. It is a plain fish butty where the bread is left unspecified and soft white is assumed, a bap across much of England, a cob through the East Midlands. The scampi version and the fish cake sandwich are the same roll filled differently. Each deserves its own article rather than being crowded in here.