Cheese and salad cream is a sandwich whose defining note is the dressing, and the dressing is specifically not mayonnaise. Salad cream is a thinner, paler, sharper emulsion: lower in oil, raised with vinegar and a touch of mustard and sugar, so it lands tangy and bright where mayonnaise lands rich and round. Spread against Cheddar that distinction is the whole sandwich. Mayonnaise would soften and pad the cheese; salad cream cuts it, the vinegar sharpness slicing through the fat the way a pickle does but as a smooth coating spread edge to edge rather than a chunky stripe. Swap the salad cream for mayonnaise and you have a different, blander sandwich. The tang is the point, not the creaminess.
The craft is the amount and the placement of an emulsion that is wetter and looser than mayonnaise. Salad cream's higher water content means a heavy hand soaks the bread faster than a stiff mayonnaise would, so it goes on as a thin, even film rather than a thick layer, enough to coat and season the whole face of the slice without pooling. It can be spread directly onto the crumb in place of butter, in which case it both dresses the sandwich and partly waterproofs it, or spread over a buttered slice for a firmer barrier and a richer base; the choice changes how sharp the result reads. The cheese is a firm sharp Cheddar cut thick enough to stand up to the vinegar tang rather than vanish under it, sliced rather than grated so it holds as a defined layer against the wet dressing. The bread is soft and plain because the salad cream is already the bright, complicated element and a strong crust would only fight it, and the sandwich is pressed lightly so the film, the cheese, and the crumb set into one thing.
The variations are mostly the dressing question repeated. Mayonnaise in its place is the milder, richer cousin and a genuinely different sandwich; a chunky pickle swaps the smooth tang for a sweet-vinegar bite; adding salad leaves turns the same dressing into the binder of a cheese salad. Each deserves its own article rather than being crowded in here.