Dear lord, I started this thread with a simple non-political question as clearly the logistice were organised some time before the election. The last few replies have been risible.
I think you will find the real answer is that we don't have a dedicated long range VIP aircraft because it "looks bad" (to whom I don't actually know given that just about every country in the world runs a widebody) . They should have just bitten the bullet years ago and gotten a 777/330 with AUX tanks and a proper VIP interior with secure coms etc and the problem would be solved. However in Australia if you do something like that then you will be destroyed by the press and the opposition. So instead we now take two aircraft.
Which is obviously a bit difficult if you want to make a big deal about climate change.