Why a post-First-World-War K3 points to 16.5-inch guns

Why might Britain reduce the K3 design's main battery from 18-inch guns?

An immediate postwar K3 would more plausibly use Britain's 16.5-inch program to trade caliber for speed or armor, while a much later version could use a 16-inch gun.

The answer first corrects the proposed reduction from 18 inches to 16 inches: immediately after the First World War, Britain was developing a 16.5-inch weapon derived from the 15-inch gun. A smaller caliber could trade some firepower for speed, armor, or another design priority. A late-1930s or Second World War version could instead use the later 16-inch development path. It also warns against projecting later high costs backward, because Britain still possessed fully staffed heavy-gun and shipbuilding infrastructure at the end of the First World War. Closing that capacity and then paying to reopen it was what made later rearmament more expensive.