Japanese carrier development without the Washington split

Would a Japan still aligned with Britain copy the Fleet Air Arm or have a British yard build its first carrier?

Japan would probably follow its historical path because its interest in naval aviation predated the treaty-era rupture, with Hōshō completed before Hermes despite Hermes being laid down first.

Japan's interest in naval aviation began before World War I and was already established before the Washington treaty settlement. Its carrier programme would therefore probably follow the historical path even if close British ties continued. Hermes was the first purpose-built carrier laid down, while Hōshō was the first completed. That distinction explains why an early Japanese carrier need not depend on a new British-yard order in this counterfactual.