The roast beef bloomer leads with the bread. A bloomer is a thick, soft-crumbed white loaf with a light, slightly chewy crust, cut into generous slices that have more give and more depth than ordinary sliced white, and that is the whole reason this version has its own name. Cold roast beef is a dense, lean, slightly dry filling, and the bloomer is the loaf that can carry it: thick enough to stand up to a heavy slab of meat without collapsing, soft enough in the crumb to fold to it rather than fight it, and substantial enough that the bread is a real part of the mouthful instead of a thin wrapper around the beef. Put the same beef on flimsy sliced white and it overwhelms the bread; put it in a bloomer and the two are in proportion.
The craft is matching the loaf to the load and managing the moisture the cold meat has lost. The bloomer is cut thick on purpose, and the beef is sliced thin and against the grain so a generous, structurally sound sandwich still folds and yields rather than going to rope. The crumb is open enough to take a smear of horseradish or a thin layer of beef dripping without turning to a sour or greasy patch, and butter to the edges seals the soft interior and bridges the salt of the beef to the wheat of the loaf. A bloomer's faint crust gives a little resistance at the edge of each bite, a quiet textural counter to meat that is otherwise uniformly tender, which is part of what the bread is contributing beyond bulk.
The variations stay inside the same loaf-led frame and mostly argue about the counter. Horseradish is the standard heat against the beef; English mustard pushes it differently; a thin dripping spread keeps it closer to the roast. The same beef on plain sliced white is the everyday baseline rather than this, the Yorkshire-pudding version is the Sunday-dinner reading, and other roast meats on a bloomer, pork with apple, lamb with mint, are their own sandwiches. Each deserves its own article rather than being crowded in here.