It's hell doing it in a oner.
Best airlines:
KLM
Emirates
Etihad
Singapore
Malaysia
If you are doing it in a oner, say with a refuelling stop in Asia, don't come back via Singapore - it's an 8 hour flight from SYD to SGP, then the most hellish 15 hours in the history of aviation from SGP to UK.
I've told DH I'm never doing the trip again without at least 3 days stopover somewhere in the middle, and for at least 4 weeks. I'm working on him to move his family (DParents/brothers and families), to UK so I'll never have to do the trip to Oz again
!
Don't get me wrong, I love Oz, but am holding out for a teleporter.....
Sydney is a fabulous city, but you may be surprised at how expensive it is - prices have rocketed in the last 15 years or so.
Don't go to Bondi, it's a hole. Get the ferry to Manly for the beach.
Oxford Street and Paddington are lovely - the houses with the iron lacework are beautiful.
It's a cliché, but lunch at Doyle's Watsons Bay on a sunny summer afternoon is bliss.
We go to the seafood markets and buy a kilo of prawns and eat them by the harbour.
Good food in Chinatown.
If you just like mooching about some of the inner city suburbs like Glebe, Balmain, Newtown are worth a look - some good cafes, restaurants and shops.
Day trips - Blue Mountains are a must. Good hiking, and the views are spectacular. You can go by train but I love driving because some of the remoter areas are the best - Hargrave's Lookout and Evan's Lookout are our favourites, and that means you can go up the Great Western Hwy and come back via Bilpin.
Hunter Valley for the scenery and the vineyards
South Coast for the beaches
Further afield for 2-3 nights, Canberra is interesting, Snowy Mountains are lovely in the summer.