Marmite on toast gives up the second slice on purpose, and the heat is what changes the spread. A single piece of bread is toasted firm, then buttered while still hot so the butter melts in, and the Marmite is scraped on thin over it. The defining fact is what the warmth does: on hot toast the near-black yeast extract loosens and thins, dragging across the buttered surface in a glossy film instead of sitting as a stiff dark layer the way it does on cold bread. The toast is no longer a wrapper holding a filling in; it is an edible plate holding a strong savoury layer up, and toasting it firm is what stops it collapsing under the melted butter and the salt of the extract.
The craft is the toast state, the order of assembly, and the scrape. The bread is taken further than it would be for a closed sandwich, crisp and dry to the centre, because it has to carry warm butter and a salty spread without going soft before the last bite. Butter goes on the moment the toast comes off the heat, so it soaks in and gives the Marmite something to slide into rather than sitting on a dry surface; the extract follows immediately, while everything is still hot enough to thin it, and is kept to a light film since the open face has no top slice to balance the salt. There is no second component. The whole effect rests on a deep, faintly bitter savouriness spread thin over hot, buttery, crisp bread, and it is eaten at once, because a single slice of Marmite toast does not survive the wait that a closed sandwich is built to survive.
The variations stay inside the open-faced savoury idea. The closed Marmite sandwich seals the same scrape between soft bread for a tin; Bovril on toast runs the same single-slice logic with a beefier note; Marmite and cheese on toast melts a tempering richness over the base; the everyday Marmite and butter sandwich keeps the crusts and the cold build. Each deserves its own article rather than being crowded in here.