I, too, wouldn't mind seeing an evening arrival option to LHR. However, the way I see it (and I am certainly not an expert in these things), such a flight would only suit passengers travelling from SYD (making the assumption that SYD is where the flight would leave from) to LHR, and would not suit passengers with connections at either end--it would have to leave SYD too early for domestic passengers to fly into SYD in time to make the flight, and would arrive in LHR too late to have useful connections to other European cities. (It may suit some passengers from other Australian cities who have had a stopover in SIN.) Ex Australia, it could work for passengers from places other than SYD if there were morning flights from major capitals to SIN to connect to the flight at SIN. This would require either additional flights, or rescheduling of flights from those cities to SIN. If additional morning flights were put on from these places, there would be a lot of QF aircraft idle in SIN. Perhaps these planes could continue on to places such as MXP, AMS and CDG (and other airports mentioned in some wish lists in other threads) instead of either lying idle in SIN or returning quickly back to the other Australian cities! It is a big capital ask, however.
Given that the evening arrival might effectively cater for only a very small category of passengers, maybe it could work with smaller aircraft, such as a 787. Maybe the use of 787s from the other cities as well would allow for some connections for evening arrivals to MXP, AMS, CDG, etc.
Also, I'm wondering what the LHR-SIN-SYD return flight times would be like.
Maybe what is needed is a good partner in Asia or the Middle East that has evening arrivals in many European cities and not just LHR as could be the case on QF metal.
Just my early thinking out loud thoughts...