The ham sandwich is the British lunchbox baseline, the sandwich every other one is measured against, and its quality is decided by three or four honest ingredients in the right proportion. Cooked ham, soft bread, butter, and one sharp counterweight, mustard, pickle, or piccalilli, is the whole thing. There is nowhere to hide: a tired ham or a mean scrape of butter shows immediately, which is exactly why it works as a standard.
The craft is the counterweight and the bread. Ham is salty and fatty, so it wants an acid or a sharp pickle to cut it, applied in a measured stripe rather than a flood that soaks the bread. The bread is soft and plain so it carries rather than competes, and butter bridges the salt to the wheat and seals the crumb against the pickle's vinegar. Sliced thin and cut on the diagonal, it asks for nothing more than to be made with care.
The variations run the cold counter. Corned beef with pickle or onion, luncheon meat and spam, ox tongue, brawn, haslet, polony, and the regional sausage sandwiches of Cumberland, Lincolnshire, and Glamorgan each swap the cure and keep the frame. Each deserves its own article rather than being crowded in here.