How Britain might answer a revealed 18-inch battleship

Would Britain build an 18-inch battleship if another navy announced one?

Possibly as a one-off, but a 16-inch battleship was the more likely fleet response.

Britain retained experience with 18-inch weapons from the First World War era and continued to consider the calibre. That made an 18-inch ship technically conceivable if another power revealed such a battleship. The more practical response was still a 16-inch design, perhaps supplemented by a one-off ship assembled in the spirit of Vanguard. In a later variation where Churchill learns Yamato's specifications before pausing capital-ship construction, the response could also bring Audacious-class carriers forward. Possessing a gun concept did not make a whole class built around it industrially or programmatically wise.