The pork pie sandwich is a finished pie put inside bread, carbohydrate twice over and unapologetic about it. A cold hand-raised pork pie is itself a complete object: seasoned pork inside a firm hot-water-crust pastry, with a layer of set savoury jelly between the meat and the wall. Sliced and laid between buttered bread, the pie does not stop being a pie; it becomes the filling of a sandwich, pastry inside more bread. That doubled starch is the defining feature, a picnic instinct that says the most portable lunch is a self-contained thing wrapped in a second portable thing.
The craft is the slice and what holds the doubled carbohydrate together. The pie is cut into rounds thick enough to keep the ring of pastry, the dome of meat, and the seam of jelly intact, because the jelly is the part that supplies moisture to an otherwise dry meeting of crust and crumb and a slice that loses it eats like sawdust. A sharp counter is structural rather than optional: English mustard or a dark pickle cuts the fat of the pork and the richness of the pastry, applied as a measured layer so it seasons without softening the bread. Butter goes edge to edge to seal the crumb and bridge the pie to the loaf. The bread is soft white, deliberately plain, because the pie brings all the structure and flavour and a crusty loaf would only fight the pastry it surrounds.
The variations stay in the picnic frame. Pork pie with piccalilli is the classic sharp partner; with a wedge of mature Cheddar alongside it becomes a ploughman's in sandwich form; a slice of cold ham added pushes it toward a full cold-cut spread. Its relatives are the pasty, bridie, and the pie-in-a-bap, the same sealed-pastry logic met in bread a different way. Those deserve their own article rather than being crowded in here.