That Airbus URV linked to earlier has the following specs:
Range: 4800 nm (not sure if that is with legal reserve, or absolute range).
Drift-up cruise altitude: 104000-114000 feet.
Fuselage capacity is 80% liquid H2 tank, 15% payload (2727 kg/6000 lbs), 5% boost rocket and retractable turbojet space.
Would take at least one fuel stop to fly LHR-SYD.