The fish finger and ketchup sandwich is the version that settles the sauce question by choosing sweetness over acid alone. Ketchup is not a neutral lubricant here; it is the single decision the sandwich is built around. Tomato ketchup brings sugar, vinegar, and a thick clinging body all at once, and that combination sits against the breaded fish in a particular way that tartare and brown sauce do not. The crumbed fish is the usual constant, a baton of white fish in a brittle orange coating, but the ketchup determines how the whole thing reads: sweeter on the front, sharp on the finish, and coated rather than merely moistened. It is the sandwich that keeps the plain, sweet, everyday register on purpose rather than reaching for a sharper sauce.
The craft is restraint with a sauce that does not want to be restrained. Ketchup is thick enough to stay where it is put, which is the point, so it goes on as a stripe across the fingers rather than a spread over the bread, where it would soak straight in and sweeten the whole slice to mush. The fingers are cooked until the crumb is brittle and laid down hot, because the coating is still the only texture the sandwich has and the sauce is the thing most likely to soften it if it is overdone. Soft white bread is buttered to the edges so a thin fat film sits between the slice and both the fish oil and the watery edge that even a thick ketchup throws. The sandwich is closed, pressed once, and eaten before the sugar-and-vinegar wet has had time to travel through the crumb.
The variations are the rest of the sauce argument, each its own sandwich. Bound caper acid is the tartare reading; a soft green layer underneath is the mushy peas reading; melted cheese forces a hot build instead of a cold one; brown sauce swaps the tomato sweetness for a darker, spiced one. The plain canonical fish finger sandwich is the form before any sauce is committed to. Each deserves its own article rather than being crowded in here.