Why Britain considered 16.5-inch rather than 16-inch guns

Why did British designers consider a 16.5-inch gun instead of matching the US Navy's 16-inch caliber?

British development had followed roughly 1.5-inch steps, a 16.5-inch weapon had already received substantial research, and one inch over the 15-inch gun offered too little return by British calculations.

Matching a rival's headline caliber differed from following an established national development path. The Royal Navy had advanced from 12-inch to 13.5-inch and then 15-inch guns, so logistics, scaling experience, research effort, and value for money favored another roughly 1.5-inch step. Moving from 13.5 to 16 inches could be worthwhile, but moving from an existing 15-inch weapon to 16 inches offered a smaller gain. A 16.5-inch design therefore occupied a more mature research position than a British 16-inch alternative while still answering American development.