Bovril on toast is the open-face version of the spread, and it gives up the second slice on purpose so the toast can act as an edible plate. Bovril is a thick, dark, intensely salty beef-extract paste, and it is far too concentrated to behave like a filling. Spread it between two slices and it reads as a smear; spread it thinly across one slice of hot toast and it melts slightly into the surface, the heat thinning it just enough to coat rather than sit. The single slice is the whole point. There is no top layer to balance it, so the entire effect rests on a strong, salty film against firm, plain bread, and the toast is no longer a wrapper holding something in but a base holding a coating up.
The craft is heat and restraint, and both are unforgiving. The toast is taken further than for a sandwich, firm and dry rather than pale and soft, so it can carry a wet, salty spread without going limp under it. Butter goes on first while the toast is still hot, and it is structural: it gives the lean extract a fat to ride on and bridges the salt to the wheat so the Bovril does not read as a flat, briny streak. The spread itself is applied thin, thinner than seems generous, because Bovril is salt and umami at full strength and a thick layer is inedible rather than richer. The bread is eaten quickly while the toast is still hot and the spread is still slack, since a cooled slice goes tacky and the coating tightens back toward paste.
The variations stay inside the strong-savoury, single-slice frame. A poached or fried egg laid over the top turns it into a small hot meal and gives the salt a soft yolk to cut it. A scrape of Marmite alongside pushes it yeasty as well as beefy. The closed sandwich version, the same spread thin between buttered bread rather than open on toast, is the portable relative built for a lunch tin rather than a plate. Each deserves its own article rather than being crowded in here.