One of the pleasantaries of a trip to Oz is the break part way through, especially if the connection is several hours, as often seems to be the case. Get something to eat, drink, have a walk around, even get a shower in the lounge if so entitled. And the several hours makes a Heathrow departure delay no real issue, and your bags will still make it. I would really feel a rapid connection a bit of a downer.
The one upside is that a number of current flights have an early morning arrival in Sydney, and hotels there seem to be more than usually reticent in letting out rooms until official check-in time, in fact more than once there's been an attitude of complete disbelief when I arrive at a major hotel at 8am. I think the times proposed here would be an evening arrival there, much more practical.