The Double Gloucester sandwich is built on a cheese chosen for being mellow rather than sharp, and that mildness is the deliberate point. Double Gloucester is a firm, smooth, buttery territorial cheese with a deep colour and a gentle, nutty flavour that never bites the way a mature Cheddar does. Put between bread it makes a different argument from the Cheddar sandwich: not a sharp cheese held in check by a pickle, but a rounded, creamy one allowed to be the quiet centre of the sandwich. The whole build follows from that. With a mild cheese the bread and any counter have to be restrained too, or they simply talk over it.
The craft is in the cut and the bridge. Double Gloucester is firm enough to slice cleanly and wants cutting thick, because its flavour is gentle and a thin shaving disappears entirely against the bread; a generous slice gives the butteriness room to register in the mouth. Butter underneath is structural as much as flavour, sealing the crumb and carrying the cheese's mild salt evenly across the slice so the bite arrives as one thing rather than bread then cheese. The bread is plain and soft, white or a light wholemeal, kept deliberately quiet because an assertive crust or a sharp loaf would bury a cheese whose whole appeal is its smoothness. Any counter is light, since the sandwich is built around the cheese being heard.
The variations are gentle by necessity, the way the cheese is. A mild chutney or a thin smear of pickle that flatters rather than fights it; a slice of ham or a little crisp lettuce for body without sharpness; the marbled version with Single Gloucester and herbs through it, shifting the look more than the register. The wider British cheese shelf, the Cheddars and the blues and the Wensleydale, each carries its own pairing logic and deserves its own article rather than being crowded in here.