London to Perth is 7813 nautical miles (great circle)
So that's 434 MPH average. Seems a bit slow.
18hrs is a scheduled block time, not a flight time, up to 30 minutes taxying out at LHR, 10 minutes taxying in at PER, flight time now down to some 17.3 hours whilst, alas, airways don't necessarily follow great circle distances ... nobody provided the airways planners with a globe and a piece of string as we used to figure out great circle tracks during another of my previous lives
