To second what others said, if you don't mind not landing in London but Paris or another EU capital, you can get cheap J fares (well, relative to QF Y+ pricing). For ex. for MEL-CDG in J mid June 2017 you can get $4K on air vietnam, $5K on air france + china southern, and $5.9K on Etihad.
Y+ on QF or J on Etihad? No choice at all, unless you need the SC.
flights.google.com if you haven't tried it before is pretty good at giving you an idea of prices at least for straight return trips, for multi-city it's not as good price wise but does show the airlines that you could use.