The meal deal sandwich is not a filling but a format, an engineered object built backwards from the chiller cabinet and the price point. It is the wedge-cut triangle in a hinged plastic carton, sold as one third of a fixed-price bundle with a drink and a snack, and almost everything about how it is made is a response to that brief. It has to survive hours flat in refrigerated air without the bread drying or the filling weeping through. It has to look the same at four in the afternoon as it did at eight in the morning. It has to be assembled fast, in volume, to a margin measured in pence. The filling matters less than the constraints, because the constraints are the same whether the triangle holds egg mayonnaise or chicken and bacon.
The craft is moisture engineering and shelf logic. The bread is a soft, slightly enriched sliced loaf chosen to resist staling in the cold, and it is buttered or spread edge to edge less for flavour than to waterproof the crumb against a filling that will sit on it for hours. Wet components are bound tight, mayonnaise-heavy or held in a thick dressing, so they do not migrate and turn the bread translucent. A leaf of lettuce often sits as a deliberate barrier between a wet filling and the bread rather than as a flavour. The triangular cut is not decoration: it shows the fill at the cabinet and lets the carton stack and face forward. Every decision serves survival in a fridge and a price, which is the whole discipline of the form.
The variations are the cabinet itself, a fixed national canon repeated across every supermarket: egg mayonnaise, chicken and bacon, tuna and sweetcorn, ham and cheese, cheese and onion, the prawn mayonnaise that sits a tier up. Each is the same engineered triangle wearing a different bound filling, and each, considered as a sandwich in its own right rather than as a chiller line, deserves its own article rather than being crowded in here.