Royal Navy planning had to cover Japan, Italy, Germany, and the Soviet Union

When did Britain begin treating Japan as a likely future enemy?

By about 1925, but Britain still had to plan against several possible enemies rather than focus exclusively on Japan.

Serious British concern about a future conflict with Japan dates to about 1925, but the threat list changed through the inter-war years. Japan and Italy became persistent concerns, while Germany and the Soviet Union also entered planning calculations. Unlike the United States, Britain could not concentrate on one Pacific scenario without exposing itself elsewhere. Royal Navy requirements were consequently broader and more compromised than a single-enemy plan.