The goat cheese sandwich is defined by its base rather than its additions, and the base is a problem the rest of the sandwich exists to solve. Soft goat cheese is bright, tart, and faintly chalky, with a lactic sharpness that reads as clean rather than rich, and on its own between two slices of bread it is one assertive note repeated. It is not a filling that fills; it is a flavour that needs framing. The whole British construction of this sandwich is the recognition that the cheese is the loud, fixed element and that everything else is chosen to give it shape: a bread soft enough not to argue with it, a fat or a sweetness to round its edge, a texture to break its smoothness.
The craft is spreading and counterbalance. Soft goat cheese is too yielding to slice, so it is spread, and spread thinly, because a thick layer reads as sour and cloying with nothing to relieve it. The bread is plain and soft so a delicate, tangy cheese is not buried under crust or competing grain, and a thin film of butter underneath both seals the crumb and softens the cheese's acidity before the first bite. Most builds add a single counterweight rather than several, because the cheese is sharp enough that two competing additions muddy it: a drizzle of honey to answer the tang with sweetness, a few toasted walnuts to answer the smoothness with crunch, a leaf of something peppery for length. The discipline is restraint, choosing one counter and letting the cheese stay the subject.
The goat cheese sandwich is the parent of a small codified family, each member named for the single variable it adds to this base. Goat cheese with beetroot trades the earthy sweetness and the staining of a root vegetable against the tang; goat cheese with red onion sets a sharp allium against it; the fig and goat cheese build of the tea tray answers the acidity with jammy fruit. Those deserve their own articles rather than being crowded in here.