Egg and cress is the sandwich that the British packaged-lunch shelf is built around, and the thing that defines this version is the cabinet, not the kitchen. The filling is the same constant as every egg sandwich, bound chopped egg with peppery mustard cress through it, but here it is engineered to hold for hours behind glass rather than to be eaten the hour it is made. That single requirement changes the build. The cress is folded into the bound egg instead of layered as a fresh green, because a layer of shoots laid against bread wilts and weeps in a chiller and a triangle has to look the same at four in the afternoon as it did at eight in the morning. The cress is still doing its job, the pepper and the fine texture against the soft egg, it is just doing it from inside the mixture.
The craft is moisture control under storage, which is a harder problem than freshness. The egg is bound tight, slightly firmer than you would make it for a sandwich eaten straight away, because mayonnaise loosens and migrates over hours and a filling that was correct at assembly goes to a slick by lunch. The bread is a soft sliced white chosen to hold its structure cold rather than for flavour, the butter or spread taken to the very edges to waterproof the crumb against a filling designed to sit against it for a working day, and the sandwich is cut into the wedge that fits the carton and shows a clean face through the film. Everything is calibrated to the worst case, the sandwich bought late in the day, not the one eaten fresh from the kitchen.
The variations are the rest of the bound-egg shelf met in the same packaged form: egg mayonnaise without the cress, egg and tomato with its bleed problem managed by a barrier, egg and bacon for the breakfast register. The made-fresh round of this same sandwich, cress green and layered, is its own thing with its own rules. Each deserves its own article rather than being crowded in here.