A bacon and sausage butty is the bacon roll with the fat doubled, and managing that fat is the whole exercise. Bacon brings rendered, salty cure; a fried British breakfast sausage brings a soft, herby, much higher-fat interior and a casing. Putting both in one fold is not simply more meat. It is two different fats and two different textures competing for the same bread, and the defining problem is grease: a butty carrying a rasher and a split sausage has roughly twice the fat load of either alone, and the build either controls that or drowns in it.
The control happens in how each protein is cooked and how it is cut. The bacon is taken until its fat has rendered out and the edge has crisped, which deliberately reduces the fat it carries into the bread and gives the soft sausage a firm, salty counter. The sausage is cooked through and almost always split lengthways and laid flat rather than left whole: a split sausage sits stable in the fold, presents a browned cut face, and stops a round sausage rolling out from between the slices on the first bite. The bread is a soft roll or soft sliced bread, chosen to absorb a controlled amount of fat without going to paste, and it is sized up because two proteins need a higher bread-to-filling ratio than one to stay structurally honest. Butter on the cut faces is still there, sealing the crumb against the combined fat bleed. The sauce question, brown or red, lands harder here than on a plain butty, because there is more fat for the vinegar or the tomato acid to cut, and a sauce earns its place by doing real work rather than just seasoning.
The variations are the rest of the full breakfast packed into the same roll. A fried egg adds a yolk and a third soft element to the fat balance; black pudding or the Scottish square sausage joins or swaps in; the regional fried carriers, Northern Irish potato bread and the Scottish tattie scone, take both proteins on a fried base instead of a roll. The brown-sauce and red-sauce camps are the single-protein roll this scales up from. Each of those deserves its own article rather than being crowded in here.