It is primarily to do with when you need to be taking off and landing during the trip. The evening flight from the UK lands in Asia mid afternoon nexy day local time, and then you can take off again 2 hours later and land when Sydney opens the next morning. A side benefit for the crew is that most people want to sleep on the first leg, and are tired and therefore docile on the second leg.
If you took the UK lunchtime flight, you would land first thing in the morning in Asia, and could then get a flight to Sydney at about 09.00. Thai (via Bangkok) and BA/ Qantas (via Singapore or Bangkok ) do have connecting flights.
80% of the flights take off UK evening time. Having done it too many times, it does fit in better with when you need to be sleeping (and the convenience of wasting less time being awake in the air), which is why most of the evening flights fill up before the daytime flight.