The sugar sandwich is the most stripped-down sweet on the British thrift shelf, and its whole character is the one thing that separates it from jam or syrup: the sugar stays a grit. Butter goes on soft white bread and white sugar is scattered straight onto it, and that is the entire sandwich. There is no second component and almost no technique. The defining fact is texture rather than flavour. A spread melts into a smooth layer; loose sugar does not, at least not at first, so the first bites are a faint crunch of dry crystals against soft bread before the grains begin to dissolve in the butter and the mouth into a sweet grain that is the whole and slightly strange pleasure of the thing.
The craft, such as it is, lives entirely in the butter and the timing. Butter is structural here in a way it is in every British thrift sweet: spread firm and to the crust, it is the tack the loose sugar presses into, and without it the grains simply fall off a dry slice and the sandwich does not exist. It also decides how fast the grit goes. Sugar on a thin cold scrape stays crystalline and crunchy for a while; sugar on a thick layer of soft butter starts dissolving on contact, the crystals slumping into a syrupy paste, so the same two ingredients give a different sandwich depending only on how the butter is laid. The salt in the butter is the only thing rounding a sweetness that is otherwise completely flat, pure sucrose with nothing else in the build to carry it. The bread is soft plain white because there is no texture in the filling but the grit and a chewy crust would be the only thing a bite has to fight.
The variations stay inside the soft, buttered, single-sweet frame and mostly move which sugar or how fine. Brown or demerara sugar brings a faint molasses note and a coarser, slower-dissolving crunch than white. Caster sugar dissolves almost at once for a smoother, less gritty version, which is nearly the syrup sandwich by another route. A scrape of condensed milk, golden syrup, or treacle replaces the grit with a smooth sweetness entirely. Each deserves its own article rather than being crowded in here.