The salad sandwich is the British plant sandwich stripped to its baseline, and it is run almost entirely by moisture management. Lettuce, tomato, cucumber, and sometimes a little raw onion go on buttered bread, often with a smear of salad cream, and that is the whole of it. There is no fat and no salt doing the heavy lifting, so the sandwich has nowhere to hide: it succeeds only if the vegetables are good and the water in them is kept off the crumb. That single problem, water against bread, is the defining fact of the form and the reason a salad sandwich is either fresh and clean or the sad, slack thing left at the bottom of the chiller.
The craft is the order of assembly and the salad cream. Tomato and cucumber are the wettest components, so they are salted and drained or patted dry and laid in the middle of the stack, away from the bread, while butter spread firm and to the edges waterproofs the crumb against whatever water gets through. Salad cream is not incidental dressing here; it is the sharp, slightly sweet acid that lifts a pile of mild vegetables out of blandness, and it is applied as a measured smear rather than a flood, because enough to season is right and enough to soak ruins the build. The lettuce goes in crisp and dry as the structural crunch, the bread stays soft and plain so it carries rather than competes, and the sandwich is made close to when it is eaten, since even a well-drained salad sandwich holds its short freshness only a little while before the leaves wilt and the bread turns.
The variations are the fillings added to the same vegetable base, each carrying the salad rather than replacing it. Cheese salad sets a wedge of Cheddar against the leaves; ham salad and chicken salad add a cold cut to the same pile; egg and tomato turn it richer. Beetroot is its own argument, a vegetable that bleeds harder than any of these and demands its own handling. Each deserves its own article rather than being crowded in here.