The plain cheese sandwich is the baseline every other sandwich in Britain is measured against, and its defining quality is that it has nowhere to hide. Sliced cheese, usually Cheddar, on bread, with butter and nothing else: there is no sauce to carry a tired ingredient, no pickle to distract from a mean cut, no second filling to share the load. Whether it is good comes down entirely to the quality of three things and the proportion between them. A block of bland factory Cheddar, a thin scrape of cold hard butter, a stale slice of bread, and the failure is total and obvious. That total exposure is the whole point of the form: it is the test, not the showpiece.
The craft is in the cut and the bridge. Cheese sliced too thin reads as nothing and dries against the bread; sliced too thick it claigs the mouth and overwhelms a plain crumb. The useful thickness is enough to have a distinct presence per bite without becoming a wedge. Butter is not optional and not garnish here: it is the only thing standing between a dry, dense cheese and a dry, plain bread, and spread to the edges it lubricates the bite and carries the cheese's salt across the slice so the sandwich reads as seasoned rather than flat. The bread is soft and plain because there is no assertive flavour in the build that wants arguing with, and it is cut and the sandwich pressed lightly so the cheese and crumb settle into one thing rather than sliding apart as it is eaten. Cheese at fridge cold is firm and waxy and mutes its own flavour, so a few minutes out of the cold before building is the difference between tasting the cheese and merely chewing it.
The variations are almost all the act of adding the one counterweight the bare version refuses. Pickle brings the vinegar cut, raw onion brings the assertive bite, salad cream brings the tangy emulsion, tomato brings the wet sweetness, and each of those swaps a name onto the same Cheddar-and-butter spine. The grated, mayonnaise-bound cheese savoury is the version that rebuilds the filling entirely rather than adding to it. Each deserves its own article rather than being crowded in here.