On Britain:
I want to keep things as close to the canon established in AtE as possible so I think making the British Isles anything but Thelemic is a retcon too far, but the idea of splitting up their homelands with smaller hostile nations is a good one. I had an idea for a joke in the context of the CKII mod where American Christians believe the Redcoats are fighting with "Presbyterian John" across the ocean, so that could be a starting point. Once the full map is in a workable state we'll see how things balance out and what is needed from a gameplay standpoint.
On Australia:
@Trudon21 has it pretty well. EU4's Australia uses the Eureka flag, so obviously that has to include Ballarat, which means BabaKiueria has to be to the north. I had spent some time thinking about divisions that might be used in an AtE-ish Australia setting but there isn't enough space on the EU4 map for a lot of them and I'm still not sure how many extra provinces ought to be added, if any.