Those listing preferable options for getting from SYD/MEL/BNE to LHR are missing the point. As Roo said, think of the entire country west of (and including) Adelaide.
QF can offer one stop flights on its own metal to London from a vast array of smaller markets. Plus direct flights to/from Perth which makes WA a more appealing destination for inbound passengers.
Plus they only need to sell ~250 tickets to fill the flight. Should certainly be possible to make it a daily service across that large market catchment.
The flight will most likely originate in MEL/SYD/BNE so will want to use the domestic terminal so they can sell all the seats on the originating leg to domestic pax with domestic connections. They will not want to have to tow it across runways to the international terminal as this wastes a lot of time/causes delays which would reduce airframe utilisation.