The prawn salad sandwich is the prawn sandwich opened up to salad vegetables, and what defines it is moisture managed across more wet components than any other prawn build carries. Small cold-water prawns are laid on bread with lettuce, cucumber, tomato, sometimes onion or cress, and a light dressing of mayonnaise or salad cream. The salad is not a garnish on the side of a prawn filling; it is half the sandwich by volume, and the prawn shares the bread with vegetables rather than dominating it. That is the structural difference from prawn mayo, where the prawns are bound and alone, and from the prawn cocktail sandwich, where the only leaf is a controlled shred of lettuce against a thick sauce.
The craft is keeping it from going wet, which is harder here than anywhere else on the prawn shelf because almost everything in it weeps. Tomato and cucumber are the worst offenders, so they are sliced and salted or drained, or kept off the bread by a butter or dressing layer that waterproofs the crumb, and the lettuce is patted dry rather than added straight from the wash. The prawns are kept whole so they hold their snap against the soft, watery vegetables instead of dissolving into them. The dressing is applied in a measured amount, enough to season and lightly bind, not enough to add to the flood that the vegetables already threaten, because a soaked prawn salad sandwich is the soggy thing left at the bottom of the chiller. The bread is plain and soft so it carries the load rather than competing with it, and it is built close to eating because the more the cut vegetables sit, the more water they shed into the crumb.
The variations narrow or shift the salad. Prawn mayo drops the vegetables and binds the prawns alone; the prawn cocktail sandwich reduces the leaf to shredded lettuce against Marie Rose; prawn and avocado swaps the watery salad for a single rich soft partner; the plain prawn sandwich removes the salad and the bind entirely and lets the prawns sit loose under butter. Each deserves its own article rather than being crowded in here.