Sandwich spread is the odd one out on the paste shelf, because it is not a pounded protein at all. It is a jar of finely chopped vegetables, gherkin, carrot, swede, onion, suspended in a thick, sweet, tangy salad-cream-style emulsion, and the defining fact is that the relish is the entire filling rather than a condiment carried by something else. The protein pastes next to it concentrate one cured meat or fish into a salty smear; sandwich spread does the opposite, taking a sweetened mustardy mayonnaise base and giving it just enough chopped vegetable to have texture and bite. It is thrift of a different kind: not a strong thing used sparingly, but a cheap, shelf-stable jar that turns plain bread into a sandwich with no second ingredient required.
The craft is the spread itself doing two jobs and the bread staying out of its way. Because the base is already an emulsion of oil, vinegar, and sugar, it lubricates and seasons in one layer, so butter is often skipped or used only as a thin waterproof seal, since the spread brings its own fat and its own acid and a buttered slice underneath can read as greasy on greasy. It is taken edge to edge so every bite is the same sweet-sharp note with the small crunch of the chopped vegetable against it, the only texture the sandwich has. The bread is soft and plain because the spread supplies all the flavour and most of the moisture, and a crust with chew would fight a filling whose pleasure is the tang and the fine vegetable bite rather than anything substantial. It is assembled cold, cut, and holds reasonably in a tin, though it slackens the bread faster than a stiff paste because it is wetter by design.
The variations are the jarred relish shelf and the additions made to it, each a small swap on the same sweet-tangy base. Sandwich spread with cheese sets a wedge against the relish; with ham it becomes a cooked-meat sandwich carrying its own pickle; piccalilli and Branston are the chunkier, sharper cousins used the same way. The protein pastes are the savoury, concentrated end of the same store-cupboard tradition. Each deserves its own article rather than being crowded in here.