Ham and cheese is the British lunchbox baseline, and the only real question it asks is which ingredient leads. Read from the ham side, this is a ham sandwich that has been given a dairy edge: a generous fold of cooked ham doing the structural work, a thin shaving of Cheddar adding salt and fat on top. Weight the layers the other way, a thick slab of cheese under one mean slice of ham, and the same name now describes a cheese sandwich with a savoury accent. The ingredients agree with each other completely; the proportion is what decides the sandwich, and the cook makes that decision every time without thinking of it as one.
The craft is keeping two ingredients that pull the same way from collapsing into one note. Cooked ham and Cheddar both bring salt and fat, so a heavy hand on both produces a rich, flat sandwich with no relief in it. The honest correction is restraint: ham folded rather than piled so it has loft and the bite gives, cheese cut with presence but not slabbed, the bread and butter left to carry rather than compete. Butter to the edges on both slices bridges the salt across the crumb and waterproofs it. Soft plain bread is the right carrier here precisely because the two fillings are loud enough on their own and an assertive loaf would only argue with them. The whole skill is proportion, which is why a bad ham and cheese is almost never bad ingredients and almost always a bad ratio.
A single sharp counter changes the sandwich entirely, and each addition has its own logic. Mustard cuts the fat with heat; pickle or chutney answers it with sweet acid; tomato adds water and a sour note that needs managing; toasting and pressing the whole thing welds it into a different sandwich again. Pease pudding, coleslaw, or a fried egg each push it somewhere else still. Each of those is its own sandwich and deserves its own article rather than being crowded in here.