See the Tabulated Record of Movements over on the Combined Fleet siteI think Yamato shows up by Jan 42. I'm not sure about Musashi and you may well be right. In any event, the Brits have at least two more modern, radar-equipped BBs than Bill projects And if we're going to 43, the Brits will get Torbeaus for land-based maritime strike, and perhaps Spitfires either aboard flattops or defending shore installations.
My own opinion is that earlier in the war, the Japanese are better off, but as this hypothetical war goes on, the Brit, for many reason, gain advantages that the Japanese don't -- better fighter direction, more new ships, getting the commonwealth resources moving in, so that much like with the USA OTL, Japan needs a quick victory.