I took a look at the Boeing website and it had the range globe, The London to Sydney Route is literally just in range. I don't think it's in range enough where they could schedule an everyday flight. Some of your assumptions were based upon the weather being right for every trip. I believe on an International Flight that you need an additional 90 minutes of fuel on board for some sort of insurance. If the London to Sydney Flight was delayed because of fog at Sydney or a Tropical Storm, the flight would probably be diverted to Canberra or Brisbane - I know the plane wouldn't have enough fuel to get to Brisbane if they were already circling Sydney. With more composites being built for airplanes and increased engine efficiency, I suspect in the next 7 to 10 years, that the London to Sydney Route will come into play.
I have to say, I was surprised that Airbus' first family of A380's didn't fulfill that need. With sheer size of the plane, you would think there there would be enough space for the extra fuel - but the you have to take that up with the extra Weight. It will be interesting if the new Boeing Dreamliner becomes the plane to take care of the London - Sydney Route.
Lastly, there economics of the route. Having Singapore right along the route of Sydney and Singapore pretty much guarantees that the planes will be full since the London to Singapore is a busy route and Singapore to Sydney is just as busy - lots of business going to between the Australians and that part of the world. Also, you have to keep 2 crews for those long hauls - not just pilots but flight attendents - per the rules dictated by the IATA. Also, you have to get the right wieght which includes the number of passenger/baggage/potable water/extra crews quarters/etc for a 24 hour flight.
I do think it will be done once the technology comes up. I don't know if another European City would want a direct flight to Sydney - remember it has to be make economic sense. If you look at the Boeing website, it mentioned the London to Perth possibilty. The 747ER had the range to do it but is there enough people to fill the route to make it worth while. I mean if you took a 25 hour flight from London to Perth - you still have to take about a 4 hour flight from Perth to Sydney. If you went through Singapore instead - you would save about 6 hours on the today journey (and having made the journey to Australia for the US - that's a big difference. Lets face it, it's not a lot of fun being on a plane for 24 hours - back in the early 90's when I flew a 767 down - our entertainment was limited (ended up drinking a lot on the way back from Brisbane to Honolulu - that actaully wasn't "a killer" - only 9 to 9 1/2 hours.