The ham sandwich is the British lunchbox baseline, the plain bar every other sandwich in the country is privately measured against, and its defining property is that it has nowhere to hide. Sliced cooked ham, soft bread, butter, and nothing else: four ingredients, no sauce to mask a fault, no heat to rescue a tired component, no crunch to distract the mouth from what it is actually tasting. Because the build is this exposed, the sandwich is decided entirely by proportion and by the quality of the ham itself. A good slice of cooked ham, properly cured and not over-watered, carried on fresh soft bread with butter spread to the edges, is one of the most satisfying things the British kitchen makes. The same sandwich built from grey, wet, reformed ham on dry bread is one of the most disappointing, and the gap between those two outcomes is the entire subject.
The craft is butter and ratio, because there is nothing else to get right. Cooked ham is salty and lightly fatty, and on its own against plain bread it reads as a single flat note, so the butter is not lubrication but the bridge that carries the salt across the slice and gives the wheat something to answer it with. The ham goes on in enough thickness to have presence and not so much that the sandwich becomes a wedge of meat, because the point of the form is balance rather than abundance. The bread is soft and plain on purpose: a crust with real chew would fight a filling that has no texture of its own, and a flavoured loaf would argue with a ham that is meant to be the whole statement. Cut on the diagonal and pressed lightly so the layers hold, it is a sandwich that asks for nothing except to be made with care, which is exactly why it functions so well as the standard against which everything else is judged.
The variations are the entire cooked-meat counter, and almost all of them are this sandwich with one thing added to give it an edge. Ham with mustard, ham with pickle, ham with piccalilli, ham with tomato, ham with salad: each takes the bare ham sandwich and answers its single note with acid, sweetness, moisture, or crunch, and each earns its own name for the swap. The honey-roast and dry-cured hams change the cure rather than the structure. Each of those deserves its own article rather than being crowded in here.