The Bovril sandwich is defined by how little of it there is. Bovril is a thick, dark beef-extract paste at the upper limit of how salty and how concentrated a spread can be, and the entire design problem is using almost none of it. Scraped thin, the way mustard is scraped rather than the way butter is spread, between two slices of soft buttered bread, a trace of it makes a sandwich out of nothing more than bread and butter. This is the closed, portable cousin of the same extract that goes on toast, and the difference is structural: the closed sandwich gives up the toast as an edible plate and instead relies on the bread staying soft and the spread staying thin enough not to overwhelm it. The defining fact is the ratio. Get the quantity wrong and there is no sandwich, only salt.
The craft is the bridge between an aggressive spread and a passive carrier. Bovril is lean, dark, and saline with no fat of its own, so the butter underneath is not optional comfort; it is the medium that carries the salt across the bread and stops the extract reading as a hard briny smear in the centre. The bread is soft and plain because the spread brings every note the sandwich has and any crust with real chew would fight a filling that has no texture to offer. It is spread to a film, not a layer, and the sandwich is pressed and cut so the thin coating is distributed across the whole face rather than pooled in one bite. Built this way it keeps in a lunch tin without weeping or staling badly, which is the practical reason the closed form exists at all.
The variations stay inside the thin-and-salty frame. A scrape of Marmite alongside runs the yeasty register against the beefy one. A few slices of cold tomato or cucumber add water and a fresh edge to cut the salt. The open-face version, Bovril spread on hot toast and eaten off a plate, is the same extract handled as a base rather than a filling. Each deserves its own article rather than being crowded in here.