The fish butty is the chip shop's fish, not the freezer's, put between bread, and that single substitution changes everything. The filling is not a crumbed finger but a piece of battered fish: a fillet of cod or haddock dropped in a wet batter and deep-fried until the coating is a hard, blistered, golden shell over steam-cooked flake. Where breadcrumb is dry and even, batter is thick, uneven, and shatteringly brittle in places and chewy in others, and the sandwich is built around that louder coating. Soft sliced white or a soft roll, butter, and not much else go around a slab of fish whose own shell carries all the texture and most of the salt. It is the takeaway portion read as a sandwich rather than a plate.
The craft is managing a coating that throws steam and grease. Battered fish comes out of the fryer with its shell sealing a pocket of very hot moisture, so it is rested briefly before it goes in the bread, long enough to stop the worst of the steam softening the crust from inside but not so long that it cools. The fillet is broken to the width of the bread rather than left whole, so it sits flat and the slice closes evenly over it. The bread is soft and plain precisely so it yields to the fish and soaks a measured amount of the grease the batter sheds, going dense underneath while the lid stays dry, and butter to the edges slows that soak just enough to keep the base intact for the few bites the thing lasts. Salt and a shake of malt vinegar go on the fish before closing, the vinegar cutting the fried richness from the inside.
The variations are the rest of the fryer between bread. The same fish on a soft Lancashire roll is the barm reading; a scampi butty swaps the fillet for breaded scampi; a fish cake sandwich uses the potato-and-fish patty instead; the saveloy and the spam fritter run a fried sausage or battered luncheon meat through the same logic. Each deserves its own article rather than being crowded in here.