EK currently runs 5 A380s per day to LHR and 2 per day to LGW. Ergo, no shortage of capacity to take over the QF flying.
Errr only partially true, EK currently operate 3 777's to LGW, although I wouldn't imagine it will be too long before 1 rotation goes the way of the 380.
I can see though that QF might think about selling the LHR slots....or at the very least, leasing them to EK.
Because of curfew restrictions at SYD, the time that QF aircraft will arrive in DXB means a minimum of 4.5 hour transit for passengers going onwards to european destination. Without the need to fly onwards to LHR, these services can be re-timed.
Again why the need to do that? why delay an onward flight based on a return sector? SYD-SIN-LHR has a broadly similar overall elapsed flight time to SYD-DXB-LHR, it's almost irrelevant where the tech stop is, so it's a simple exercise based on existing timings to merely schedule the LHR departure to arrive eventually in SYD after curfew ends....just like they do now?
A quick check of EK's schedule shows that it's about a 2 hr transit on the SYD-LHR using EK413-001(Europe is only 1 hr ahead of the UK).
So the curfew argument holds no weight as QF1 is scheduled out of SYD at a bit after 5pm local time SYD.