How earlier 18-inch battleships might alter Yamato

Would an earlier British 18-inch-gun battleship cause Japan to arm Yamato with 20-inch guns?

Probably not; it would more likely force Japan to rethink the whole programme once 18-inch guns no longer supplied a unique advantage.

Japan probably would not answer an earlier British 18-inch battleship simply by moving to a 20-inch gun. The larger consequence is that 18-inch weapons would no longer give Yamato a unique qualitative advantage. If Britain had such a ship, the United States might also develop an 18-inch response. Japan could then reconsider Yamato and Musashi entirely or seek a different way to offset American numbers. The counterfactual focuses on how a rival's earlier innovation changes the purpose and gun calibre of a later programme.