The red camp is usually framed as the default, the choice people make without thinking, and that framing undersells it. Bacon and red sauce is a deliberate position in a national disagreement, and tomato ketchup does something to a bacon roll that brown sauce specifically does not. Ketchup is a tomato base with a high, clean sugar level and a bright acetic-acid edge, thicker and glossier than the malt-and-tamarind alternative. Laid against salt-cured bacon, that combination reads as a direct, almost confectionery contrast: the sugar amplifies the salt instead of arguing with it, and the tomato adds a fruit-acid note the brown bottle cannot. It is the sweeter, simpler, louder answer to the same question, and the people who take it red consider that loudness the point rather than a fault.
The build turns on the fact that ketchup behaves better in a roll than its rival. It is thick and clings, so it can be spread across the cut face without immediately running, which lets it go on more generously and stay put through the eating. The bacon is still cooked until the fat has rendered and the edge has firmed, because soft bacon under a sweet, thick sauce reads as one heavy, sugary note with nothing to break it, and the rendered, salty crisp is what the ketchup is there to play against. Butter under the sauce is doing structural work: it seals the soft floured roll against the tomato's acid and moisture so the bread holds rather than going translucent. The roll is chosen soft so it yields to the bite and soaks a controlled amount of fat, the design constraint of a sandwich made fast and eaten on the move. The sauce is layered to the edges rather than dolloped in the middle, so every bite carries the same balance instead of one dry mouthful and one sweet flood.
The variations are the opposing case and the breakfast around it. Bacon and brown sauce is the other camp, the identical roll making the contrary argument, and the no-sauce purists sit outside the disagreement entirely. Sausage joins or stands in for the bacon, a fried egg brings a yolk that the sweet sauce has to negotiate, and the regional fried carriers, the Northern Irish potato bread and the Scottish tattie scone, take the red bottle through a different base. Each of those deserves its own article rather than being crowded in here.