I am curious what is the fastest option to fly from London (UK) to Sydney (Australia). Basically I want to spend less time getting from A to B.
Shortest duration is London to Bangkok to Sydney which comes in at 20 1/2 hours, plus connection time in practice. This is only a tiny bit shorter than via Hanoi (20:40) or Saigon (20:50). The connection time will vary depending on the schedule that day but expect a minimum of 45mins to an hour that is dependent on airline policy for a particular airport.
Thai Airways does the route themselves but you can also use two airlines such as Thai, Emirates or Qantas for the LHR-BKK segment and Thai, EVA or BA for the BKK-SYD segment.
You can get the distances from Great Circle Mapper which totals at 17,091km. This is extremely direct as the distance between London and Sydney is 17,016km.
Actually this shouldn't be to hard to find. Google flights makes it easy for you to see duration of your entire journey as well as each individual flight and the stopover time. (as well as many other flight booking engines)
Shortest flights are around 22h30m but you can save +100USD by choosing a journey that takes 25 minutes more. Off course that is on the dates I picked randomly and true today but will probably be different in a few hours/days as well as for other dates.
Stops on the shortest routes are either Dubai (DXB), Singapore (SIN) or Bangkok (BKK).
I included a screenshot of the search result with google flights. The list is sorted by duration (shortest first). The complete list is longer but I needed to zoom out to a level that made the results unreadable to be able to include the complete result.
As of March 2018, the quickest route between London and Sydney is now via Perth, as it is approximately 5-10 minutes quicker than the previous fastest link between Singapore and Bangkok.
However this difference is due to a shorter stopover as in pure flight time Bangkok comes out ahead with 20h 15m in the air versus 20h 45m for Singapore and 20h 50m for Perth.
It is possible that on a given day either of these two might beat coming via Perth if the stopover time is reduced, and Qantas is currently ahead purely because they probably have customers wanting to fly LHR-SYD as one of their key use cases to optimize for.
Answer to the current version of the question:
The shortest duration will can change regularly as the airlines adjust schedules. So this is only really answerable by knowing the exact travel dates and what the OP counts as duration, flight time/travel time/connection time, etc.
Answer to the previous version of the question:
There are shorter routings available, such as LHR-HKG-SYD, which is only 1 Nautical Mile longer than a direct route.
Note, the upcoming LHR-PER-SYD may become a contender, at least for convenience, to most of Australia.