Why the RAF has little early reach

What role would the RAF play in this war against Japan?

Very little at first: its 1939 bombers lack the range and forward bases needed to reach Japan, so substantial RAF involvement must wait for the Allies to secure and supply closer airfields.

Theoretical aircraft range is only one part of an operational air campaign. Even Hong Kong is too far away for the kind of heavy-bomber reach the scenario would require, and bases farther north would need to be captured, built, supplied, and protected. The opening campaign therefore remains predominantly naval, with substantial RAF involvement growing only after the fleet establishes the logistics for it. American land-based aircraft in the Philippines are better positioned to contribute earlier because their airfields and supply position are already closer to the likely operations around Hong Kong.