Britain could imagine a larger step from 13.5-inch guns, but the 15-inch choice better matched existing development paths and production realities. Elswick, Armstrong Whitworth, Beardmore, Coventry Ordnance Works, the Royal Gun Factory, and Vickers formed the industrial base available for heavy-gun development. The number of capable makers reduced production risk, while a 16.5-inch weapon would have demanded a larger research and manufacturing leap. The practical test was whether a weapon could be produced in time for ships that might have to fight.
Notable Point · 42:38-45:20
Why the 15-inch gun was the practical leap
The gun-selection section treats the 15-inch Mark I as an industrially practical leap rather than the largest possible British gun.