Roll and bacon is the Scottish morning sandwich phrased in the Scottish way, bread first. South of the border the same thing is a bacon bap, a butty, a barm or a sarnie depending on the town, but in Scotland the order of the words is the point: the roll is named before the filling because the roll is what fixes the identity. A morning roll is a soft, floury, slightly chewy bap baked to be eaten the same day, and roll and bacon is that specific roll carrying rashers, not any bread with bacon in it. The construction is plain on purpose. Rashers, a soft roll, butter, and one sauce, and the whole thing turns on how the bacon and the roll meet.
The craft is grease management inside a short-lived bread. The bacon is cooked hard enough to render its fat and crisp its edge, because a limp underdone rasher gives the roll nothing but salt and water, while a rasher with a browned edge gives it the salt-fat note the soft crumb is there to absorb. The roll is dense enough to take a little of that rendered fat without collapsing and soft enough to compress to the bacon rather than tear, which is exactly the property the filling needs, and it is filled and eaten within the morning because it stales fast. Butter to the edges is not optional: it bridges the salt of the bacon to the plain wheat and seals the crumb for the short time the sandwich exists. The sauce, brown or red, is the genuine Scottish divide and goes inside in a measured stripe so it cuts the fat without running through.
The variations are whatever shares the roll with the bacon. A fried egg adds a yolk that has to be kept from flooding the crumb; a tattie scone turns it into a fuller stack; black pudding or Lorne brings a second savoury slab alongside the rashers. The closest neighbour is roll and sausage, the same roll with a slab or a link of sausage where the bacon would be, and that is its own sandwich. Each deserves its own article rather than being crowded in here.