The missed opportunity for a common improved British 15-inch gun

What was a major British missed opportunity during the naval-treaty era?

An improved common 15-inch gun for Nelson, Rodney, and older capital ships could have made fleet-wide upgrades easier.

The discussion distinguishes a missed opportunity from an outright mistake and selects the F3 design's proposed 15-inch armament as a British example. A better 15-inch gun for Nelson and Rodney could have supplied a reason to upgrade the batteries of older British capital ships. The expected benefit was better performance than the existing 15-inch/42 weapon. A 15-inch/50 proposal is mentioned, alongside uncertainty about a 48-calibre alternative and a later 45-calibre option. The argument favours the opportunity for improvement across the existing 15-inch fleet.