Prawn mayo is the bound default of the prawn shelf, and what defines it is the bind itself rather than the prawn. Small cold-water prawns are folded into mayonnaise until they just hold together, seasoned, and spread on soft bread. The mayonnaise is not a dressing sitting alongside the prawns; it is the matrix that coats every one of them and turns a loose, slippery handful of shellfish into a single cohesive filling that stays on the bread instead of rolling off the back. This is the supermarket and sandwich-bar reading of the prawn sandwich, the one most people mean when they say prawn sandwich at all, and the whole sandwich is decided in the bowl before it ever meets a slice.
The craft is the ratio. Too much mayonnaise and the prawns are lost in a slick that soaks the bread and reads as cold fat with a faint seafood note; too little and the filling is dry and falls out the open edge in a heap. The correct amount is just enough to lacquer each prawn and let them cling to one another without pooling, and it is unforgiving because there is no salad and no sharp counter in the plain build to distract from a bind that is wrong. The prawns are kept whole rather than chopped so the snap survives the coating, and a little seasoning, lemon, salt, sometimes a turn of pepper or a trace of cayenne, is worked through to stop the mayonnaise reading as bland. The bread is soft and plain so it carries rather than competes, and because the mayonnaise already seals the crumb the butter step is often dropped entirely; where it stays it is thin, since the bind is doing the lubricating work. The filling is spread evenly so each bite is the same, because an uneven prawn mayo is mostly bread at one end and a wet clump at the other.
The variations move the bind toward other readings. The prawn cocktail sandwich tints the mayonnaise into Marie Rose and adds lettuce for the retro version; prawn and Marie Rose makes that sauce the lead rather than a tint; the prawn salad sandwich loosens the bind and adds salad vegetables for moisture and crunch; prawn and avocado partners the bound prawns with a rich soft fat; the plain prawn sandwich drops the bind entirely and lets the prawns sit loose under butter. Each deserves its own article rather than being crowded in here.