Corned beef and pickle is the corned beef sandwich resolved with vinegar, and the pickle is the part that makes it work. Tinned corned beef is salt-cured beef pressed into a dense, fatty brick that across a whole sandwich tires the palate on its own one savoury register. A spoonful of Branston or another sweet, dark, vinegary vegetable pickle cuts straight through that richness with acid and sugar at once and resets the mouth between bites. The relief here is sweet and sour, not the raw pungency that onion brings; the pickle softens and rounds the salt of the beef rather than fighting it with another sharp edge, and that is the defining character of this version against its onion sibling.
The craft is the quantity of pickle and the defence of the bread. Branston and its relatives are wet, sharp, and sweet, and a generous smear poured straight onto the crumb soaks through to a sour patch within minutes. The pickle goes on in a measured stripe, and the corned beef, cold and firm, slab or flaked, is laid as a barrier between the pickle and at least one face of the bread so the moisture has somewhere to sit other than the loaf. Butter to the edges on both slices is the real waterproofing, sealing the crumb against the pickle's vinegar so the sandwich survives the gap between assembly and eating. The chunky diced vegetable in a smooth pickle base also gives a slight bite against the dense meat, which is part of what the pairing is reaching for: texture as well as acid.
The variations are mostly a question of which jar against the same brick. A sharper piccalilli pushes mustard and vinegar harder and turns the sandwich brighter and hotter. A chunky homemade chutney swaps the register again toward fruit. Corned beef with raw onion replaces the sweet acid with a raw allium bite, and corned beef hash leaves cold cutting agents behind for fried potato. Each of those is its own sandwich and deserves its own article rather than being crowded in here.