The streaky bacon butty is the national morning sandwich with one variable deliberately changed: the cut of the bacon. The constant is the bacon butty itself, a soft buttered roll or slices around hot bacon with a stripe of brown or red sauce; what makes this version itself is that the rasher is streaky, cut from the pork belly, rather than the back bacon most of Britain reaches for. The defining fact is the fat. Back bacon is cut from the loin and is mostly lean meat with a band of fat along one edge, so it stays meaty and pliable when cooked. Streaky is ribboned with fat all the way through, and rendered hard it does not stay soft, it crisps, shattering into a salty, brittle layer. The whole character of this butty follows from that single substitution.
The craft is rendering the fat fully and building the roll around a crisp filling rather than a tender one. Streaky has to be cooked further than back bacon, long enough for the fat ribbons to render out and the rasher to set crisp, because half-cooked streaky is flabby and the entire reason to choose this cut is the crunch. That changes how the bread behaves: where a soft back rasher beds quietly into a roll, crisp streaky wants a roll that yields against it so the contrast holds, and it sheds rendered fat that the bread soaks, which is closer to the chip butty's fat-and-bread logic than back bacon ever gets. Butter still bridges the salt to the wheat, and the sauce question, brown or red, is the same national divide applied inside so it does not run. The roll is soft and plain because the filling is now the loud, brittle, salty element and the bread is its foil.
The variations stay inside the breakfast-roll frame and mostly negotiate the cut and the crisp. Back bacon swapped in is the meatier, more pliable default this version defines itself against. Smoked or dry-cured streaky deepens the salt and the savour without changing the texture. A fried egg lays a soft yolk over the brittle rasher; American-style candied or maple streaky pushes the fat toward sweetness. Each deserves its own article rather than being crowded in here.