British Trident warheads, missiles, and decoys

Does Britain independently build the warheads carried by its Trident submarines, and why might one missile carry fewer warheads than its capacity?

The warheads are British-built while the missile system is American, and a missile can trade warhead count for penetration aids and decoys within broadly comparable arms-control responsibilities.

The British-built nuclear warhead is distinct from the American-supplied Trident missile system that carries it. A missile need not be loaded with the greatest number of warheads it can physically accommodate. Reducing that number can leave room for decoys and other aids intended to help the payload penetrate defenses. Britain and the United States also operate within broadly similar arms-control responsibilities, so the issue goes beyond a technical maximum. Platform, delivery system, warhead, and penetration package are separate parts of the weapon system.