Garuda Indonesia is currently selling an excellent £1850 Business Class deal from London Heathrow to Australia (Sydney or Perth) for January to July 2019.
The earliest travel date I can find outbound is Friday 11th January so, whilst good for a Winter break, isn’t any use if you’re planning to travel over Christmas 2018.
Garuda is about to end its direct service from Heathrow. Instead, you will be flying London to Amsterdam (Garuda codeshare on KLM), Amsterdam to Jakarta and Jakarta to Sydney / Perth.
Garuda is a SkyTeam member so you can pick up a lot of miles in your preferred SkyTeam frequent flyer scheme, potentially Delta SkyMiles because you would also get the ability to redeem on Virgin Atlantic. Etihad Guest is also an option. (Do check your exact booking class as wheretocredit.com.)
The Garuda home page is here. They do not fly every day so if your search throws up an error then move forward a day. Once you have a price up you can easily scroll back and forth seven days at a time to find the best fare.
Your best option to maximise your miles when paying is American Express Preferred Rewards Gold. This offers double points – 2 per £1 – when you when you book flight tickets directly with an airline. Our review of Amex Gold is here.