oh yeah, the Brit fleet.
Japs go after Brits. Assume they get 3 transport 1 tank.
Sea zone east of Japan has battleship and 4 transports. (Assume Kwangtung transport killed) The most UK will have available to attack that is transport, destroyer, carrier, fighter, and bomber on UK2.
If UK used its air up for the UK turn (that is, leaving them out of threatening range of Japan’s navy, very possible), what actually threatens that Jap battleship and 4 transports is the UK carrier and destroyer. That’s fine. Japan has a free hit, and after that, inexpensive transports die.
If UK moved its transport, fighter, and bomber east to threaten Japan’s sea zones, that leaves immediate holes in UK’s game, key being Anglo-Egypt. This creates its own set of problems for UK. Japan is still threatened, but Japan has plenty of ways of dealing with that threat, one of which involves leaving Pearl Harbor alone to allow more Japanese units to wipe out the UK fleet.
So that addresses the question of 1) splitting the UK fleet, and 2) using the UK fleet aggressively early. The third case, unification of the UK fleet southwest of Australia (or thereabouts) also leaves UK with a set of problems, but is also addressable by Japan. Since that UK move is not immediately threatening, Japan can carry out Pearl, but Japan will have to race back to deal with UK, which can be a real pain if UK fighters flew east from London.