The faggot sandwich is defined by the gravy, not the meat. A faggot is a coarse, peppery ball of minced pork offal, liver and heart and lights bound with onion and breadcrumb, traditionally cooked and served swimming in a dark, glossy onion gravy. To put it in bread is to take a wet, hot, sauced thing and ask a slice to contain it, which makes this one of the more structurally precarious sandwiches in the British repertoire. The faggot supplies an intense, mineral, well-seasoned filling, but it is the gravy that decides whether the sandwich works or simply disintegrates in the hand.
The craft is moisture control under a filling that is, by its nature, soaked. The faggots are sliced or split rather than left whole, both so they sit flat and hold their place and so the bread is not asked to bridge a sphere. The gravy is the variable everything else answers to: served loose over the plate it makes a knife-and-fork dish, but committed inside a sandwich it has to be reduced thicker and used in a controlled quantity, enough to keep the offal moist and carry its onion depth, not so much that it floods straight through the crumb. A soft white roll is the honest carrier here, and crucially it is meant to absorb: unlike most sandwiches, where soak is the enemy, a faggot bap expects the bread on the gravy side to drink a little of the sauce and become part of the eating. Butter still goes on, less for waterproofing than to set a faint barrier and a salted floor under a filling whose pepper and liver want a steady base.
The variations are mostly the gravy and what sits beside the offal. Mushy peas alongside push it toward the full plated faggots and peas served as a roll; a thicker, sweeter onion gravy leans it richer; a version with the faggot left whole and pressed is the messier, more rustic reading. Each tips the sandwich toward a named build with its own logic, and those deserve their own articles rather than being crowded in here.