The all-day breakfast sandwich is an act of compression. A cooked breakfast is a plated meal of separate components eaten with a knife and fork in sequence, and this sandwich takes that whole spread and forces it to behave as one handheld thing. Bacon, sausage, a fried egg, and often a spoon of beans have to be reorganised so they hold together between bread instead of sitting in their own corners of a plate. The defining decision is not which items go in but how they are made to stop being a breakfast and start being a sandwich.
The craft is almost entirely about moisture and ordering. Beans are the structural problem: a wet ladle of them will pass straight through the bread before the sandwich reaches the mouth, so they are drained tight or kept to a thin layer with the sausage split flat beneath them as a barrier. The egg is the second hazard, and a runny yolk that works beautifully on a plate becomes a flood in the hand, which is why the all-day version usually sets the yolk harder than a cooked breakfast would tolerate. The bacon does the load-bearing flavour, the sausage is butterflied so it lies flat rather than rolling, and the bread is soft enough to compress around an uneven, lumpy filling without splitting. It is a sandwich built to be eaten standing up, sold by cafes and vans from opening until close, which is the entire reason it exists rather than the breakfast it copies.
The variations are mostly a question of how much of the fry-up survives the translation. A leaner build keeps to bacon, sausage, and egg and reads as a tidy breakfast roll; a maximal one adds black pudding, hash brown, fried bread, and mushrooms and starts to fight its own structure. Regional breakfasts each push their own components into the same fold, and those deserve their own articles rather than being crowded in here.