Hi
Just dropping into say I'm sorry you are in this situation but I wanted to (unfortunately) confirm that your suspicions about having to remain in the country are likely right.
My ex left me 4 years ago (he is Australian) after I'd been living here for less than 2 years, with no career history, with a 2 year old and all family/support/network etc, in the UK (and having talked about returning to the UK). I consulting several relocation legal specialists, as my ex would not permit me to return, and the gist of it is that without documented physical or sexual abuse (and sometimes not even in those cases) there is little change of winning a case. The law over here for divorce is no fault (doesn't matter who has done what to whom) and they start at 50/50 care if a parent wants that and works backwards depending on desire, age, location, circumstances.
I'm not saying this to make you alarmed just so you can make an appropriate plan. If you want to go back to the UK, frankly you need to engineer a way to go back as a family and then leave.
It hugely hugely sucks. I won't hijack but I was left in a dreadful financial/career/housing position and could write a book about the perceived injustice of it but I do know its good to be armed with knowledge. Of course, get your own legal advise, but well meaning UK friends still ask me if I could do x or y as things are different in the UK about divorce/child support/location than over here (which is well meaning, but drives me nuts).
Feel free to PM me. BTW its nearly 4 years later and with a few claw marks where I've dug in deep, things are certainly on the up and up and I've learnt to love Australia and where I live but I would not wish the process on anyone so do get in touch if I can help.
Good luck xx