The Lancashire sandwich is decided by a property most cheese sandwiches never have to think about: this cheese crumbles rather than slices. Traditional crumbly Lancashire is loose, moist, and faintly tangy, a cheese that breaks into soft flakes under a knife instead of cutting into a clean slab. That changes how the whole sandwich is built. You do not lay a sheet of it across the bread; you crumble it on, and the filling becomes a scattered, lactic, lightly sour layer rather than a solid plank. The defining fact of the build is that texture: a sandwich made to a cheese that will not behave like a slice has to be assembled differently, and the result tastes and eats differently because of it.
The craft is distribution and the counter. Because the cheese crumbles, it has to be pressed gently into the buttered bread so it stays put rather than spilling out of the sides at the first bite, and it is spread evenly so a bite is not all cheese at one end and bare bread at the other, which is the failure mode a crumbly filling invites. The tang of the cheese is mild and slightly sour, so the classic counter is restrained, a measured stripe of pickle or chutney that meets the lactic note without burying it, and the butter underneath both bridges the cheese to the bread and seals the crumb against the pickle's vinegar. The bread is plain and soft because crumbly Lancashire at its best is gentle and a little sour, and an assertive loaf would simply talk over it. There is a creamier, firmer Lancashire that slices more readily; the crumbly type is the one that gives this sandwich its character.
The variations stay on the cheese shelf and keep the regional dairy as the point. Crumbly Lancashire with Branston or a fruit chutney is the everyday form; creamy Lancashire cut thicker behaves more like a standard cheese slice; tasty Lancashire, the aged and sharper grade, pushes the tang further. Each of those deserves its own article rather than being crowded in here.