Ok I flew back with Qantas in February. Flying there did the Ads - Sing - heathrow- Coprnhagen. Was not impressed with heathrow airport at all, Terminal 5 was ok ...ONCE you got through the screening gates, too few for transferring passengers.
Coming back I flew the shortest route possible.
heathrow-Bangkok 10 1/2 hours flight
Bangkok - Sydney 8 1/2 hours
Sydney - Ads 2 hours.
Time spent off plane in Bangkok about 1 1/2 hours
At Sydney check through the Qantas int to Qantas domestic transfer after immigration and they bus you to domestic terminal and take your bags.
Time in Sydney airport about 2 hours.
heathrow terminal 3 is awful, so overcrowded like a cattleyard. Bought something to eat and struggled to find somewhere to sit, not fun with heavy cabin luggage haha.
This is the shortest route available, all other routes have longer flight times and longer stops at airports.
Here's another tip ...on Qantas A380s seat 71D has no seat infront of it as this is where the crew quarters escape hatch is, so you get extra legroom without having to pay the exit row cost. You just have to pay the seat choice fee.