Because "Brits" does not equate to "London".
QF seem to point to cost as the pure reason that passengers are deserting them in droves for the ME carriers (and soon to be Chinese); that is one factor but just as important is the convenience of a "local airport". Not everyone wants a 6hr drive to LHR, pay exhorbitant parking charges etc. Similarly, not everyone looks forward to sitting on the M25 in the morning peak having just flown in from Australia and with another 200 miles of driving ahead.
EK long ago realised that if you can attack the UK regional market with cheap airfares, that the market will later withstand prices similar to QF's once you've established the regional identity.
QF complain that they cannot sustain the UK destinations that the likes of EK can - and they shouldn't be expected to. What they never appeared to do was play EK at their own game of discounting to a regional airport and then building up their prices once they'd established that market. May be they never had the right aircraft for SIN/GLA, afterall it appears they haven't the right aircraft for LHR given their partial withdrawal from that market.
Regards,
BD