Tuna and sweetcorn takes the bound tuna baseline and answers it with a sweet pop, and in Britain this is the default rather than a variation. The tinned tuna and its measured mayonnaise bind are the constant; the variable is tinned or frozen sweetcorn, and what it adds is a small bright burst of sugar and a firm skin that bursts between the teeth. Against a savoury, slightly heavy tuna mix the corn does two things at once: it lifts the flavour with a counter-sweetness and it breaks the soft uniformity of the filling with a regular pop. So embedded is this pairing in British lunch habit that for many people a tuna sandwich simply means this one, and plain tuna mayo is the deviation.
The craft is the same bind problem as the baseline with the corn folded in late and dry. The sweetcorn is drained well, because the liquid in the tin is more hidden water that slackens the bind and soaks the bread from within, and it is stirred through at the end so the kernels stay whole and distinct rather than crushed into the mix and lost. Proportion is the quiet decision: enough corn that most bites carry a pop, not so much that the sandwich reads as sweet and the tuna becomes a background note. The tuna is bound to the same just-cohesive point as the baseline, the bread is soft and plain, and the filling is spread evenly so the kernels are distributed corner to corner rather than pooled at one end.
The variations are the rest of the tuna family, each leading on a different counter to the same fixed bind: cucumber for cool water-crisp, onion for raw bite, celery and pepper for the branded crunch, plain mayo for the baseline. Sweetcorn is the one that chose sweetness and became the national norm, and adding a little red onion or pepper to it pushes it toward the crunch version. Each deserves its own article rather than being crowded in here.