The ploughman's sandwich is a pub plate compressed into bread. The ploughman's lunch is an arrangement, not a dish: a wedge of cheese, a pickle, an onion, some bread and butter, often a slice of ham and a piece of fruit, laid out separately and assembled bite by bite by the eater. The sandwich takes that deconstructed meal and closes it, putting the whole plate between two slices so the components arrive together rather than side by side. The defining move is the compression itself, the decision to make the eater's job, the building of each forkful, into a single fixed object.
The craft is fitting a plate's worth of contrast into a sandwich that still holds. The cheese is the structure: a thick slab of firm Cheddar cut to have presence, because a grated scatter would lose the wedge that defines the lunch. The pickle is the engine, a sharp dark chutney or Branston that the cheese is robust enough to stand up to, spread in a measured layer rather than a flood that soaks the bread. Raw or pickled onion is the crunch a soft filling needs, and butter under the cheese seals the crumb against the pickle's vinegar. The bread is crusty rather than soft plain loaf, a bloomer or a granary, because this is a substantial filling and a slack bread would collapse under cheese, pickle, and onion together.
The variations follow the plate it came from. Ham folded in beside the cheese makes it a fuller ploughman's; a tart apple sliced thin brings the fruit from the lunch into the sandwich; pork pie standing in for the ham pushes it toward a picnic build. Its parent is the wider British cheese sandwich, the same firm cheese and sharp counter handled without the whole-plate ambition, and the regional cheeses run the same logic each in their own right. Those deserve their own article rather than being crowded in here.