Why theater geography matters when swapping the RN and USN

If the Royal Navy fought Japan and the US Navy fought Germany and Italy, which fleet would be better positioned?

Probably the Royal Navy, because Pacific range can be supported logistically, while the confined Mediterranean conflicts more sharply with American design assumptions.

The fleet-swap counterfactual leaves geography unchanged and gives the Royal Navy the more manageable adaptation problem. British operations across the Pacific would require additional tanker support, while grouped carriers, night strikes and night-fighting methods offer ways to use its existing capabilities. Washington and London treaty limits on individual ships and total tonnage had forced both navies to tailor their designs to expected operating areas. Total allowances could require smaller cruisers to obtain enough hulls and affect carrier choices between protection and aircraft capacity. American ships would face particular difficulty in the Mediterranean, where confined waters limit their ability to leave the range of land-based aircraft. Both fleets would need to develop tactics suited to their own ships in the new theatre.