Sorry just because it might be mandated doesn't mean people do it, there is little to no enforcement. Not like what we have in HKG, foreign governments should take note and follow the lead of the HKG government when it comes to enforcing isolation/quarantine.
The LHR flight is producing a lot of covid positive arrivals in to hkg and this will need to be addressed if the route doesn't want to be shut down; the solution is government testing on departure at LHR. It seems the remote testing or testing 72 hours before departure is either inaccurate or allowing people to contract the virus in the window prior to the flight, surely preflight testing for all pax at the point of departure is the way that air travel can get going again before we all have a vaccine.