In this instance your best bet would be to keep an eye out for QF33 to see if that becomes available either on day of travel to LHR or alternatively on the day before.
As mentioned earlier, you could always try and rock up to Sydney international around noon and see if First check in can switch you onto that. Again there is no guarantee they will do that. If they don't you'll end up spending more time at the airport waiting for your PER bound flight (thank goodness for lounge access). However, if they do then you'll have access to the far nicer F lounge plus a better on board service (plus arriving into PER sooner). It's a gamble here. You have to consider whether it is worth potentially spending some extra time at the airport for the opportunity to get onto that earlier flight with better lounge access and onboard service. Certainly for me, I'd look like a fool had I been turned away by First check in and my partner and I had to sit around the airport for an extra hour or two.
-RooFlyer88