Ham and mustard is the ham sandwich with the mustard treated as the cut, not the seasoning. Cooked ham is salty and faintly sweet, and a lean fold of it on buttered white bread is pleasant and slightly monotone. English mustard is the loud counter: hot, sharp, volatile, the kind of heat that clears the sinuses and recedes fast rather than lingering. A thin scrape of it through a ham sandwich does not flavour the ham so much as interrupt it, snapping the sweet salty register with a jolt of heat on a bite-by-bite basis. That deliberate interruption is the defining character of this version; the mustard is the reason it has its own name rather than being a plain ham sandwich.
The craft is the quantity of mustard and the order it goes in. English mustard is far stronger than a Dijon or an American yellow, so it is spread thin and even, a film rather than a stripe, because a heavy hand turns the whole sandwich into raw heat and buries the ham it was meant to lift. It goes on the bread, under the meat, so the heat is distributed across every bite rather than sitting in a hot pocket, and butter still goes to the edges underneath it to bridge the salt and seal the crumb. The ham is sliced thin and folded for loft rather than slabbed, because the contrast the build wants is sharp heat against tender mild meat, and a dense plug of ham flattens that. Soft plain bread carries it; the mustard is doing all the work that needs doing.
The variations mostly swap the heat. Wholegrain mustard trades the clean burn for a milder, seedy, more textured bite; Dijon goes smoother and less aggressive; honey mustard pulls the whole thing sweet and tames it. Ham and pickle answers the same richness with acid instead of heat, and ham and cheese adds fat the mustard then has to cut. Each of those is its own sandwich and deserves its own article rather than being crowded in here.