How quickly might the Royal Navy reach 18-inch battleships?

Without naval treaties, how long would it take the Royal Navy to build fast battleships armed with twelve 18-inch guns?

Probably by the early 1920s, after the preceding capital ships had driven the necessary dockyard expansion and established the design lineage.

The starting point is a fleet with all four Admiral-class ships converted to carriers and four fast battleships armed with twelve 15-inch guns. Supporting those eight large ships would already require expanded docks and facilities around the world. That investment makes a successor with twelve 18-inch guns plausible in the next generation, with an estimate in the early 1920s. An expanded N3-style design illustrates the proposal, though its arrangement is disputed and two turrets forward with two aft is suggested as a likelier layout. The estimate depends on the earlier ships and their infrastructure already being under construction.