As far as actually dividing up the southern Pacific Islands between ANZAC and the UK goes, I tend just to try and take them all with ANZAC; Take Java turn 1, then Dutch New Guinea on round 2 (I usually consider a DOW against Japan with ANZAC in round 2 a very good idea financially, since, assuming Malaya hasn’t fallen, it gets you an extra 5 IPCs during you collect income phase that you wouldn’t otherwise have if you let Japan declare war on you in round 3 (which they’ll do anyway), and if New Britain, the Solomon Islands, and New Guinea are still under Allied control (which is probably likely, seeing as Japan has to go real far out of its way to take any of them), that makes Dutch New Guinea worth, in essence, 5 IPCs, as opposed to the 4 IPCs you get from Sumatra), and Sumatra on round 3.
If you play this right, ANZAC should (typically) have collected 14 + 24 + 28 = 66 IPCs by the end of round, which gives you a total of 76 IPCs available to spend by the Purchase Units Phase of round 4, which can get Anzac 1 fighter, 2 aircraft carriers, a Battleship, and a destroyer (Navy consists of 1 cruiser, 2 destroyer, 1 battleship, 2 aircraft carriers, and 4 fighters). That’s a pretty strong fleet for such a small power, and if Japan doesn’t turn its attention toward Sydney immediately, you can take the 30 IPCs you’ve got in round 5 and make that 1 crusier, 2 destroyers, 2 battleships, 2 aircraft carriers, 4 fighters, and a submarine.
I guess the Pacific UK could take them, but honestly, here’s how I see it: UK is supposed to take Japan on land, and ANZAC is supposed to back the US up in the sea. The problem with the UK taking all the money, is that unless the UK player is fighting on land and in the sea simultaneously (which, in essence, wastes ANZAC’s entire income for the Allies, since you’re basically exhausting another power’s income to fill a spot ANZAC was practically made for), if the UK takes the southern Pacific isles, Japan will just turn around and take them all back; at least with ANZAC, if you play your cards right, you’ll have an impressive navy to use as a deterrent by the time Japan shows up.
That, and I also just really, really like ANZAC, haha - it’s probably my favorite power in the game, so I usually do my best to make sure it turns into a powerhouse anyway (which, as you can see, does have its strategic advantages).