The Primula cheese sandwich is named for one specific product because the product, and the way it is dispensed, is the entire sandwich. This is not a generic processed-cheese build that happens to use Primula; it is the soft, spreadable cheese that comes from a squeezable tube, mild and faintly tangy and engineered to flow under finger pressure, piped or smeared straight onto soft white bread and nothing else. The tube is part of the character as much as the cheese is: the spread is formulated to extrude in a smooth, even line and to lay down without any of the dragging or crumbling a cut cheese gives, and that frictionless softness chosen on purpose is the whole point. There is no butter, because the spread carries its own fat. There is no sharp counter, because the appeal is one gentle, undemanding thing.
The craft is almost the refusal of craft, which is its own kind of reliability. The spread is formulated to stay stable: it will not split into oil and curd, will not weep, and will not sharpen however long it sits warm in a school bag, so the sandwich survives a morning in a way a cut-cheese one does not. Squeezed from the tube it lays down edge to edge with no resistance and no need for a knife, and that thin even layer lightly seals the crumb so the bread stays soft rather than drying out by the interval. Pressed flat and cut into triangles or fingers, it holds together because there is nothing wet inside to break it. The honest version is plain soft bread and a generous run of the tube, made in well under a minute and eaten without ceremony, the novelty of the dispensing being half the reason it gets made at all.
The variations stay inside the soft, mild frame because the format rejects anything assertive. A slice of cucumber or a little grated carrot for crunch without sharpness; a thin layer of ham turning a snack into a proper lunch; a smear of the tube on a cracker rather than bread. The wider processed-cheese-spread sandwich read from a tub, and the foil-triangle build, are its near relatives, sharing the soft mild logic without the squeezable tube. Each deserves its own article rather than being crowded in here.