The 14-inch King George V battery was a government choice

Did the naval treaties force the 1939 King George V class to use 14-inch guns?

No; the British government chose the calibre in hopes that other powers would follow, rather than the Royal Navy selecting it as its preferred solution.

An audience comment says that Britain became stuck with 14-inch guns on the King George V class. The answer attributes this to the British government rather than the Royal Navy. The government expected that building a 14-inch battleship would encourage other states to follow the smaller calibre. In this reading, that hope misjudged the competitive politics of battleship size. The exchange separates treaty diplomacy and civilian policy from the navy's own technical preference.