Well, in my group I am the Jap player. And so far I have found the best turn 1 move for me is 2 Minor Complexes in asia turn 1, this way starting turn 2 I can pump out mech and tanks and quickly overwhelming china, leaving UK and ANZAC no choice but to attack me, keeping the US out and poor as long as possible, and giving me turns 3 and 4 to spend all my cash on ships. Once I do this and take my combined pacific fleet to hawaii and now US is in big trouble.
Now for the point of this. I have played myself a couple of times and spent a while thinking what is good for UK in this case. IF japan doesnt grab those money islands from anzac and uk, and time is there, build a navy, same with anzac. have all fighters from the 2 of them go to defensive locations and hold the line. If the 2 of them can bring together a dozen or so ships that can move in tandem (remember that these 2 both go before japan goes again), it forces japan to split its navy so that they dont reach northern china or japan, and puts japan on the defensive giving US the time to maneuver a sizeable fleet into striking distance, go after islands, and in general put a hurt on japan. Again this relies on letting japan do its thing and not provoking them. If japan is agressive this wont work.