Building on my previous comments, one of the reasons I find the international impostor theory so difficult to imagine is that there would have to be at least one woman involved, independently or working with a man, more likely, if we are going with the whole, "let's fly to England and get married, but also not get married because we can't legally do that within the time frame, so we will just do some sightseeing here together for no real reason other than to make your family believe that you are in England when we could have done that from Aus" scenario. (I'm not making fun of anyone who thinks this, by the way, I still don't rule it out completely.)
So the woman is either from the UK, from Aus, or from another country altogether. She has to be the one to travel back on Florabella's passport, regardless of whether the man accompanies her.
If she's from another country than the UK and enters using her own, real identity, the passport that she entered the UK with will register as never having left, meaning that her travel Visa will have expired at some point within the UK. This is insanely illegal, and could have led to a block being put on her passport. So she would have to be using another fake passport, or just not care.
Wherever she was travelling from, she would have Florabella's passport. Then, presuming that she wants to ever leave the country again, she would need her own passport. And then, she would also need to have Marion's documents with her to carry out the bank withdrawals.
None of this is impossible, but the STAKES involved in this would be HUGE. It's not technically a crime to carry identification for other people in your luggage, but if security had checked her bags there would have been A LOT of questions about having documents with three different names, all supposedly the same person. If they were professional criminals, which they would have to be to pull this off, I cannot see them taking such a huge risk when it would have been FAR easier to conduct the crime in Aus and just give the impression that Marion had left for England without her actually going.
I've explained this poorly, I think, but that's just my two cents.