Chicken and lemon is one member of the chicken-mayonnaise tea family, and what separates it from its siblings is a single aromatic: bright acid worked all the way through the bind. Poached or roast chicken is chopped and held in mayonnaise, the same as in any tea-tray chicken sandwich, but here lemon zest and a little juice are folded into the dressing itself rather than dripped over the top. The acid is not a garnish on the side of a mild filling; it is dissolved into the bind so that every forkful of the chicken carries the same clean, citrus lift. That integration is the whole sandwich. Take the lemon out and you have plain chicken mayonnaise, which is a different and flatter thing.
The craft is calibrating the acid against the bind without breaking either. Too much juice and the mayonnaise loosens, slackens, and weeps into the bread; too little and the lemon reads as nothing at all against the fat. The fix is to lean on zest, which carries the citrus oils and aroma without the water, and to use only enough juice to brighten, so the dressing stays thick enough to hold the chicken in a cohesive layer. The chicken is cooked gently and kept moist, because lemon is a sharpener and dry meat under a sharp dressing tastes drier still. The bread is soft, plain, and buttered to the edges so the crumb is sealed against a filling that is, by design, slightly wet, and so nothing competes with a flavour built on lightness. Cut crustless and small, it is a sandwich meant to taste fresh rather than rich.
The variations are its own family, each defined by swapping that one aromatic. Chicken and tarragon trades the bright acid for an aniseed herb worked through the same bind; chicken and watercress sets a peppery green against the meat instead; coronation chicken pushes the dressing toward spice and fruit. Each deserves its own article rather than being crowded in here.