In order to go through some of Japan’s basic moves I’m going to have to make some assumptions since there are quite a lot of variations they can see due to the play out of R1 thru UK 1. So I’ll assume any bid went to Germany, Russia attacked some combination of Wrus, Belo or Ukr, Germany took Egy, cleared out the Med of all Allied ships, and countered what it could in Europe, while the UK landed in Alg, countered Egy (or setup up an Afr counter in Rd 2).
The UK presents an interesting dilemma for the Allies because their empire is spread out over the entire world. So you want to use a strategy that you think will maximize those units (already on the board) potential while you allow yourself the time to build up your army/navy from London and move out from there.
The first thing you need to realize is you are not alone in the battle. It will take all 3 Allies to beat the Axis, but due to other priorities you can't really expect significant help from the UK and US until round 4 or 5 (outside of some air support). So, what is a good way to set up a solid base and ensure your survival until the Allies can take some pressure off of Moscow?