Read through the bacon butty rather than the salad bowl, bacon and avocado is a bacon sandwich in which the avocado has been recruited to do the job the sauce normally does. The bacon is the point: rendered, salty, crisp at the edge, the load-bearing flavour the whole thing is organised around. The avocado is not a co-filling competing for the centre but the cool, fatty layer that stands in for the brown sauce or the butter, smoothing the bacon's salt and supplying the soft counter that a butty's bread and fat usually provide. That inversion is the defining idea of this reading, and it changes the proportions: more bacon, less green, the meat in charge.
The craft is the same heat and grease problem every breakfast butty solves, with one substitution. The bacon is cooked to render its fat and crisp its edge, then laid into a soft roll or soft bread that can absorb a little of that fat without disintegrating, exactly as a bacon butty would. What changes is the cool element. Instead of butter and a stripe of sauce, a layer of crushed or sliced avocado is spread against the bread, where it does three jobs at once: it lubricates the way butter would, it carries the bacon's salt the way the sauce would, and it adds a richness the plain butty does not have. The avocado therefore goes on the bread side and the bacon on top, so the meat keeps its crispness and the avocado keeps it from reading as a dry, salty mouthful. It is a quick build for a morning counter, made to be eaten hot and at once, because warm bacon and ripe avocado both reward speed.
The variations sit on the butty's own shelf: an egg added for body, a stripe of sauce kept alongside the avocado, the roll swapped for the regional word for it. The salad-led reading, where the avocado is the body and the bacon a seasoning, is a different sandwich and deserves its own article rather than being crowded in here.