Why Royal Navy manpower differed from Japan's

Would the Royal Navy face the same manpower problem as the Imperial Japanese Navy in an expanded fleet?

Britain could draw on the Dominions and empire and, crucially, on a much deeper civilian maritime industry with widespread practical technical knowledge.

Both navies would face constraints, but Britain had more ways to mitigate them through the Dominions and empire. Britain's huge maritime industry and infrastructure also created a deep pool of practical knowledge. Even without assuming a dramatically better school system, that civilian base provided more people familiar with marine technology and operations from whom the Royal Navy could recruit and train.