Simon, welcome to the forum. Despite what I am about to write, your investigation through AFF is most certainly the absolute best in your circumstances - this place is full of the best ideas.
I want to start with some stern realities - getting 4 seats in a premium cabin is always hard, especially in peak periods. Understand this as a base, so you are realistic, but also beleive there is some chance and thus hold hope
I am getting out of up-to-date advice, as things change very quickly. And also, take my advice (as with anyone) in that it is an attempt to help, but suffers from my own knowledge-restriction.
To attain sufficient points to get 4x return premium flights to the UK with Qantas is a big task. The amount you would have to spend on frequent flyer cards, etc is astronomical. You can earn points in a million ways, but it is a very slow process. For example, if I book an Avis car rental for a week for $500 and get 500 points, that is not even worth my time to put in my FF number - it is about $5 in true value.
With your goals I would be much more inclined to research other FF programs where you can actually buy points. (you can buy points with many, but usually that is a very expensive game). Have a look at Lifemiles. They very regularly allow you to buy points at huge discounts. They also offer specials where you can transfer points (for a cost) from one account to another with 100% plus bonuses. Lifemiles used to (am not up to date) have very good availability on certain Australia-Europe routes. So you could fly something like Thai in F to Paris for the cash cost of only a couple of thousand dollars return. I am vaguely aware that their site is harder to use, but if I was in your situation I would be seriously looking at these sort of things. Getting a connection from Paris to the UK is easy and cheap.
Others here will have far more experience in other programs. But i insist, if you want multiple bookings every year, you should investigate buying points rather than trying to earn them.
At least you have a solid and practical Life experience in good and bad. You managed to emigrate from the UK
But you ended up in Perth