The jam sandwich is the canonical British thrift sweet, and the thing that actually makes it work is not the jam but the butter underneath it. Strawberry or raspberry jam goes onto soft white bread that has first been buttered to the edges, and that butter is doing structural work, not flavour work. Jam is wet and acidic, and left to sit straight on a slice it bleeds into the crumb until the bread is a damp pink rag by the time anyone eats it. The butter is a waterproof barrier between the two. It is the single decision that turns a thing that falls apart into a sandwich that survives a satchel until lunch, and it is the reason the build has lasted as the default sweet of the British kitchen rather than a soggy failure.
The craft is the barrier and the ratio. Butter is spread firm and right to the crust so there is no unsealed margin for the jam to find, and the jam goes on in a measured layer over it rather than a thick pour, because too much slides under the top slice and squeezes out the sides the moment the sandwich is pressed flat. The salt in the butter is not incidental either: it rounds the flat sweetness of the jam so the filling reads as fruit rather than as sugar. The bread is soft plain white on purpose, because the filling has no texture of its own and a chewy crust would be the only thing in the sandwich a bite has to fight. Cut on the diagonal and pressed, it is a sandwich engineered for a lunchbox, not a plate.
The variations stay inside the soft, sweet, buttered frame and mostly move the fruit or the ratio. A tarter fruit, blackcurrant or damson, leans harder on the salt to balance it. More jam and a scrape of butter tips it toward cloying and toward the bleed the build exists to prevent. The Scottish jam piece, the same idea carried in its own dialect and tenement identity, and the cream-tea reading with clotted cream alongside, each deserve their own article rather than being crowded in here.