The kipper sandwich is built around a fish so strong that the entire job of the bread is to stand out of its way. A kipper is a herring split, salted, and cold-smoked until it is oily, dark, and intensely savoury, a flavour that arrives with no subtlety and asks for none. Boned and laid on plain bread and butter, it needs nothing added because anything added competes and loses. The defining decision of this sandwich is restraint enforced by the ingredient: a kipper dressed up is a kipper buried, so the canonical build is deliberately close to bare, the bread and butter functioning as a plain foil rather than a partner.
The craft is boning, fat, and a neutral base. A kipper is full of fine pin bones, so the work that matters happens before the sandwich is built: the fish is filleted carefully so a bite is flesh and not a mouthful of bones, which is the single thing that separates a good kipper sandwich from an unpleasant one. The smoke and oil are assertive and slightly drying, so butter spread to the edges is the one element that earns its place, bridging the salt of the fish to the wheat of the bread and carrying its richness rather than masking it. A squeeze of lemon is the only acid the traditional build wants, doing the job it does on any oily fish, cutting through and lifting the smoke. The bread is plain, soft or lightly toasted, and never assertive, because a loaf with real character would argue with a filling that already says everything.
The variations stay close to the fish and the breakfast register it comes from. A warm kipper, grilled and put hot between bread and butter, is the fuller breakfast reading. Smoked mackerel through the same plain treatment is the oilier neighbouring build. A scrape of horseradish or a grind of pepper is as far as the dressing sensibly goes. Each of those deserves its own article rather than being crowded in here.