The chicken salad sandwich turns on a question the name leaves open: is the chicken bound or is it not. In the British reading there are two distinct sandwiches sharing one label. One is sliced or shredded chicken laid loose on bread with salad vegetables alongside it, dressed lightly if at all. The other is chicken already bound in mayonnaise as a salad of its own, with the lettuce and cucumber added around it. They eat completely differently, and which one a shop hands over is rarely stated. The defining fact of this sandwich is that ambiguity, and the better versions resolve it deliberately rather than by accident.
The craft, in either reading, is moisture management against a filling that has very little fat of its own. Poached or roast chicken is lean and dries fast, so the loose version depends on good bread, a butter or mayonnaise layer for lubrication, and salad that brings water-crisp without bleeding, while the bound version depends on just enough mayonnaise to make the meat cohere without slumping into a slick. The salad components are the recurring failure point: tomato and cucumber weep, lettuce wilts, and once any of them sit against the crumb the sandwich goes wet from the inside. The fix is the same as it always is here, butter spread to the edges as a barrier, vegetables salted and drained or kept dry and added late, the bread soft and plain so it carries rather than competes with a deliberately mild filling. Cut and eaten reasonably fresh, it should taste clean and lightly savoury, the chicken lifted by the salad rather than lost under dressing.
The variations are the rest of the cold-chicken shelf, each defined by which single element is set against the meat. The coronation chicken sandwich adds curried, fruited mayonnaise; chicken and watercress leans on a peppery leaf; the cheese salad and ham salad sandwiches keep the same loose-or-bound salad frame around a different protein. Each deserves its own article rather than being crowded in here.