Given MEL-LHR distance is almost identical to SYD-LHR what's the chances that Melbourne would get a direct Service from QF as well as Sydeny?
Zero.
Stuff Qantas, the sydeny centric airline. I flew business class via Etihad/KLM/Malaysian to Europe from Melbourne last month and couldn't be happier. We got great deals on the price, the service and punctuality were excellent and there was the added satisfaction of QF not seeing a dollar of our money.
To put that another way, why the heck would anyone want to sit in an economy seat for 18+ hours without a break? Especially with Qantas levels of cabin (non) service? The airframe might do the distance, First and Business class might not mind, but economy?