The site is a six-pronged shipyard stretching 850 feet across and 170 feet into the air, and it stands as a kind of watchkeeper over Barrow-in-Furness, a small town of about 67,000 people in the U.K.

Welcome to Barrow-in-Furness: the forgotten town behind the U.K.’s nuclear defense seeking resurrection. At any given moment, a Vanguard-class submarine is somewhere in the world’s oceans, carrying up ...