Why 18 inches approaches the practical limit for battleship guns

Would Britain have escalated from 18-inch to 20-inch battleship guns?

Probably not, because the shell, mounting, barrel and ammunition penalties make 20 inches far less practical than 18 inches.

A 20-inch or 508-millimetre weapon looks impressive on paper but would require an exceptionally heavy shell, enormous handling machinery and a very large barrel. Those demands reduce the number of rounds a ship can carry and complicate every part of the mounting. Eighteen inches is therefore close to the upper edge at which a battleship gun remains operationally useful rather than merely spectacular.