The distance from London to Sydney is 17.000km. The current flight airspeed record is 3.500km/h. I would doubt that anything close to 8.500km/h is physically possible without using rockets. That's the speed of the fastest missile.
"U.K.-based Reaction Engines is developing technology for Synergetic Air-Breathing Rocket Engines (SABRE), which could one day allow aircraft to fly up to five times faster than the speed of sound — that’s Mach 5 or 3,836 miles per hour.
At that speed, hypersonic flights between London and Australia could be over in just four-and-a-half hours."
https://www.cnbc.com/2019/04/08/hypersonic-flight-technology...
"FLIGHTS FROM UK-AUSTRALIA COULD TAKE JUST FOUR HOURS BY 2030"