The haslet sandwich is built on a cooked pork loaf, not a slice of ham, and the difference is the point. Haslet is a Lincolnshire baked meatloaf of seasoned minced pork, traditionally including offcuts and offal, bound with breadcrumb or rusk and pushed hard towards sage and black pepper, then cooled and sliced cold. What it brings to bread that cooked ham does not is seasoning carried inside the meat itself: where ham is a salt cure that wants an external sharp counter, haslet arrives already herbed and peppery, a savoury loaf that is its own statement. The defining decision of the sandwich is to treat it that way, as a regional cooked meat whose flavour is internal, rather than as another mild slice to be dressed up.
The craft is the slicing and the restraint. Haslet is soft, fine-grained, and a touch crumbly because it is a bound minced loaf rather than a muscle cut, so it is sliced thick enough to hold together between two slices of bread and laid in flat so it does not break apart under the press. The bread is kept soft and plain so it yields to a tender filling rather than compressing it into paste, and butter on the bread does the structural bridging, carrying the loaf's pepper and sage across the crumb. Crucially, the sandwich asks for very little on top of the meat: the sage and pepper are already there, so a heavy condiment fights the seasoning rather than completing it, and the most this build usually wants is a thin smear of mustard or a little pickle as a sharpener, applied as a stripe rather than a flood. It is not heated; the reading that matters is cool, herbed, soft loaf against plain crumb.
The variations stay inside the cold-sliced, internally-seasoned frame, and most are a question of which cooked loaf is on the bread. A mustard or pickle alongside is the common sharpening swap rather than a different sandwich. Polony, brawn, and the regional cooked sausages of Cumberland and Glamorgan share the logic of a pre-seasoned meat that slices cold and needs little from the build. Each deserves its own article rather than being crowded in here.