The crisp sandwich is the plainest statement of a single idea: a crunch trapped against soft bread. A handful of potato crisps is laid on buttered white bread, the second slice goes on, and it is pressed flat. There is no protein, no sauce, no second thought. What makes it a sandwich worth naming rather than a joke is the contrast it manufactures from nothing, a hard, brittle, salty layer pinned between two yielding faces, so that one bite delivers shatter and give at the same time. Taken under its bare generic name, this is the phenomenon itself, stated without affection or regional dressing.
The craft, such as it is, lives in the butter and the press. Butter does the structural work: it grips the crumb and holds the loose crisps so the filling does not cascade out the open edge, and it adds the fat a dry crisp on dry bread has none of. The press is the technique. Pushing down hard breaks the crisps in place, which binds a loose heap into a single layer that holds for a few bites and produces the crunch the whole thing exists to deliver. The bread is chosen soft and plain on purpose so it compresses to the shards rather than fighting them, and timing is the one real constraint, because the crunch dies fast once the crisps sit against buttered bread and start to soften. It is built and eaten close together for exactly that reason.
The variations are entirely in the bag. Cheese and onion brings a savoury coating, salt and vinegar a sharp acidic edge, prawn cocktail a sweet one, and the strongly flavoured branded crisps are a national in-joke that nonetheless obeys the same internal logic. Each changes only the seasoning while the structure holds, and each deserves its own article rather than being crowded in here.