Except that flights from HKG & SIN also arrive around 18:00 in normal times, and with 0-2 flights per day, and (I guess) without passengers, it shouldn't matter if they choose those arrival times rather than the crack of dawn.
I'm struggling to remember when either BA or VS last had a schedule that involved pm arrivals from either HKG or SIN.
Looking at their timetables for this time last year:
VS207 HKG-LHR STA 04:30
BA032 HKG-LHR STA 04:50
BA016 SIN-LHR STA 05:05
BA028 HKG-LHR STA 05:30
BA012 SIN-LHR STA 05:50
Though VS does currently have an ad-hoc arrival from HKG at around 14:00 some days.