Naming a bacon roll after its sauce is a confession that the sauce is the argument. Bacon and brown sauce is the brown camp stating its case in a national disagreement that splits households, building sites, and breakfast counters straight down the middle. Brown sauce is a fermented, malt-vinegar and tamarind base carrying date and molasses sweetness, with a thinner, more savoury, slightly sour register than the red alternative. Against salt-cured bacon that is its specific virtue: the vinegar and tamarind cut the rendered fat rather than sitting on top of it, and the spiced, dark sweetness reads as a counterweight to the meat instead of a sugary partner to it. The bacon and the bread are the constant. The brown bottle is the variable, and the people who reach for it will tell you it is the only one that belongs anywhere near a rasher.
The build is engineered around keeping that sauce where it is put. Brown sauce is thinner than ketchup and runs faster, so it goes on the cut face of a soft floured roll and inside the fold, never on the outside, where it would slide off before the first bite. The bacon is taken far enough that the fat has rendered and the edges have firmed, because a thin, sharp sauce needs crisp salt to push against rather than soft, wet meat that it would simply soak into. Butter still goes on the roll under the sauce: it seals the crumb against the vinegar and stops the bread going through to paste before it reaches the hand. The roll is soft on purpose, absorbing a measured amount of fat and sauce without collapsing, which is what makes this a thing eaten standing up at half seven in the morning rather than a thing requiring a plate. The sauce is applied as a stripe, not a flood, because brown sauce that has pooled is brown sauce that is now running down a wrist.
The variations are the rest of the disagreement and the breakfast it sits inside. The red camp answers with bacon and red sauce, the same roll arguing the opposite case, and a meaningful number of people quietly take it with no sauce at all and consider the question beneath them. Sausage joins or replaces the bacon, a fried egg adds a yolk that complicates the sauce question, and the regional carriers, the Northern Irish potato bread and the Scottish tattie scone, route the same brown bottle through a different fried base. Each of those deserves its own article rather than being crowded in here.