The chip butty with curry sauce is, at its plainest, chips and chip-shop curry sauce folded into bread, and the version that matters here is the one made over the counter rather than at home. At the chippy the sauce is already there, a tub of thickened, mild, faintly sweet curry gravy held hot beside the fryer, and the butty is built to order with the chips still spitting from the basket. That sequence is the defining fact of the counter version: hot chip onto soft bread, a ladle of hot sauce over, the bread closed and pressed while everything is still at frying heat. The home build, sauce from a tin reheated on the hob and poured over chips that have already started to cool, is the same idea at lower temperature and lower stakes. The counter one is eaten standing up, in paper, on the walk; it is a sandwich with no interval between assembly and the first bite.
The craft is heat held long enough for the sauce, the chip and the butter to become one wet filling rather than three things sitting apart. Chip-shop curry sauce is built thick on purpose, closer to a gravy set with starch than a thin sauce, because a thin one would run straight through soft white bread and out of the fold before the second bite. Poured hot over hot chips it slackens just enough to coat them and then thickens again as the whole thing cools in the hand, gluing the loose chips into something that holds its shape. The butter underneath is not redundant under the sauce: it goes on first, waterproofs the crumb against the gravy soaking through, and carries a little salt across the bread that the mild sauce on its own leaves flat. The bread is plain pillowy white because the filling is soft, wet and intensely savoury, and a crust with real chew would only fight a sandwich that has no firm element in it anywhere.
The variations are the rest of the chippy condiment rack applied to the same chip-and-bread base. Gravy instead of curry sauce is the Northern build with a meatier, less sweet wetting agent. Mushy peas instead make a soft green bed under the chips rather than a sauce over them. Cheese melted into the hot chips before the sauce goes on is the loaded version, and gravy and curry sauce together is the half-and-half a few counters will do if asked. Each deserves its own article rather than being crowded in here.