The cheese and onion crisp sandwich exists for one sensation: the crunch of crisps trapped against soft bread, and the cheese-and-onion seasoning is the reason this particular packet is the one chosen. There is no cheese and no onion in it as ingredients. The flavour is entirely the dusting on the crisps, a savoury, slightly sharp, dairy-and-allium powder that reads as cheese and onion the moment it hits the tongue. The sandwich is a packet of crisps, soft buttered white bread, and the bread pressed down so the crisps shatter under the hand. The whole pleasure is the violent textural contrast between something that crunches and something that yields, and the seasoned crisp supplies both the crunch and the flavour at once.
The craft, such as it is, is timing and the press, because the crunch is on a fast clock. The crisps have to go in dry and be eaten soon, since they soften the moment the butter's fat and the bread's moisture reach them, and a crisp sandwich left to sit becomes a stale, greasy disappointment with none of the snap that was the entire reason for making it. Butter spread thick and to the edges is structural rather than incidental: it is the only binder, gluing the loose crisps to the slice so the sandwich holds together for a few bites, and its salt sits under a snack that is already salted so the build does not read as one flat savoury note. The bread is deliberately the softest, plainest white available, because the contrast only works if the bread offers no resistance of its own, and the press is the defining action: a firm push down crushes the crisps into a single brittle sheet so the sandwich bites cleanly rather than collapsing into a bag of fragments.
The variations are simply the crisp aisle, and they are a national in-joke with real internal logic. Salt and vinegar swaps the dairy note for a sharp acid one against the bread; ready salted strips it to pure crunch and salt; the strongly seasoned and the branded shapes each change the flavour while keeping the texture argument identical. The chip butty is the same starch-on-starch instinct made hot and soft instead of cold and crisp. Each deserves its own article rather than being crowded in here.