The prawn sandwich, taken under its bare generic name, is the plain reading of cold small prawns between bread, and what defines it is how little stands between the shellfish and the loaf. Small cold-water prawns, sweet and faintly briny and no bigger than a thumbnail, are laid on soft buttered bread, and in the most stripped version that is the whole sandwich: prawns, butter, bread, a squeeze of lemon if anything at all. There is no bound dressing in this reading and no salad bulk. The discipline is the same one a good crab sandwich runs on, which is to get out of the way and let a sweet, delicate prawn be the entire statement rather than burying it under a sauce that does the talking instead.
The craft is the butter and the restraint around it. A small prawn carries very little fat of its own and almost no acid, so the butter is structural as well as flavour: spread to the edges, it bridges the prawns to the crumb, supplies the richness the prawn lacks, and lightly waterproofs the bread against the moisture the prawns weep as they sit. The prawns are left whole and loose rather than chopped, because the slight resistance and the clean snap of a whole prawn is the texture that makes it read as shellfish rather than a smooth filling. Lemon, when it is used, goes on as a squeeze and not folded through, so it lifts the sweetness without slackening the butter. The bread is plain and soft on purpose, because a good prawn is quiet and an assertive crusty loaf would simply drown it. Brown bread is the usual choice for the faint nuttiness that flatters a sweet prawn, and it is built and eaten close together because a naked prawn sandwich does not hold the way a sealed, heavily dressed one does.
The variations are the rest of the prawn shelf, each leading on something this plain reading deliberately leaves out. Prawn mayo binds the prawns in mayonnaise rather than leaving them loose under butter; the prawn cocktail sandwich adds Marie Rose sauce and lettuce for the retro reading; prawn and Marie Rose makes that sauce the whole point; prawn and avocado brings a rich soft partner; the prawn salad sandwich folds in salad vegetables; the prawn cocktail crisp sandwich abandons the prawn entirely for a flavoured crisp. Each deserves its own article rather than being crowded in here.