QFi is not my preferred carrier - I like WY from and to MNL - but a proposed 0800 hours departure from LHR at least allows passengers to arrive at the airport on the Elizabeth line as the latter's first train arrives Heathrow at about 0500 hours.
However 0800 is too early to allow passengers travelling by train from say Birmingham, Edinburgh, Glasgow, Leeds, Liverpool, Manchester or Sheffield to connect, and ditto for the minority who fly within the UK.
An 0500 arrival ex SYD in LHR may be OK for businessmen wanting an early start for the day but is a horrible time if you're a leisure traveller as most hotels don't allow check-in until 1500 or so (granted, one can be lucky with a sympathetic desk clerk on the odd occasion if a room was unoccupied the previous night). The thought of arriving in central London by say 0645 on a February morning doesn't excite.
In an ideal world if the slot was available, a 1200 'high noon' departure from LHR would be better but could mean lower aircraft utilisation. QFi might also be worried about the prospect of late running from Heathrow meaning the aircraft fails to 'make' curfew in Sydney the next night.