There was a one off flight to test what such a flight would be like but it required taking a non-commercially low number of passengers.also thought there was a direct LHR to SYD or did i dream that?
If PER-LHR can’t be made to work, SIN-LHR could work or even perhaps DRW-LHR.
Remember SIN is expected to be the next travel bubble so I think travel from LHR via SIN would be acceptable to the government later this year if travel direct from LHR to Australia would be.