The chip bap puts the soft roll under the chips on purpose, and the bap is the half of the sandwich that is actually being chosen. A bap is a round white roll with a yielding crumb and a pale, faintly floured top, baked to be pillowy rather than crusty. It tears rather than cracks, gives under a thumb and slowly comes back. That softness is doing real work here, because a chip sandwich is starch on starch with no protein to brace it, and a crusty roll would fight the chips for the bite while a soft bap surrenders to them and lets the filling lead. The chips, the butter, and the press inside are the constant every chip sandwich shares; the bap is what makes this the soft-roll reading of it.
The craft is heat, fat, and a controlled amount of absorption. The chips have to go in hot, because the whole sauce of a chip sandwich is butter melting on contact with the potato into a slick that binds bread and filling, and a cold chip will not make it. The bap is buttered to the edges while still cool so that emulsion forms inside the sandwich rather than soaking straight through the floured top, and the lower crumb is left to take a measured amount of fat and steam and go slightly dense and translucent while the lid stays dry and chalky. The chips are laid in a flat layer rather than a heap so the roll closes evenly, then the bap is pressed down so its loose crumb collapses around the chips and binds them into something that holds for a few bites instead of a roll with potato sliding out of it. Salt and vinegar, or a stripe of brown sauce or ketchup, goes on the chips before closing so it does not run down the outside.
The same sandwich answers to other words a few counties in any direction. It is a barm in Lancashire, a cob through the East Midlands, a butty wherever the bread is left unspecified and soft sliced white is assumed. Add chip-shop curry sauce, gravy, or mushy peas and it becomes a meal rather than a snack. Those deserve their own articles rather than being crowded in here.