The fish finger and tartare sauce sandwich is defined by the cut its sauce makes. Tartare is not a smooth condiment; it is a bound sauce with grit in it, a mayonnaise base carrying chopped capers, gherkins, and a sharp edge of lemon or vinegar. That construction is the whole point. The crumbed fish is the usual constant, a baton of white fish in a brittle orange coating, but tartare does something a plain sauce cannot: it answers the fish with a built-in acid and a scatter of small, sour, briny pieces that break up an otherwise rich and uniform mouthful. Where ketchup sweetens and brown sauce darkens, tartare cuts. It is the version made by people who want the sauce to argue with the fish rather than coat it.
The craft is in the sauce holding its structure against a hot filling. Tartare is emulsified, so it stays as a thick, clinging layer rather than running, which is exactly what is wanted: it goes on the fingers as a band, not spread thin over the bread where it would soak in and lose its bite. The capers and gherkins keep their texture even against warm fish, so the contrast survives the first bite, and the lemon or vinegar in the base lifts the fish without thinning the sauce to a slick. The fingers are cooked until the crumb is brittle and laid down hot, because the coating is still the sandwich's only texture and the sauce is the thing most likely to soften it if it is overdone. Soft white bread, buttered to the edges, keeps the base from wicking up fish oil and the sauce's faint moisture before the thing is eaten.
The variations are the rest of the sauce argument made differently. Sweet tomato is the ketchup reading; a darker spiced sauce is the brown sauce reading; a soft pulse layer underneath is the mushy peas reading; melted cheese pushes it from cold to hot. The plain canonical fish finger sandwich is the form before any sauce is settled on. Each deserves its own article rather than being crowded in here.