The Sydney / London flight most direct path cross Russia and that's already 17000km so probably it has to go south of Iran and that's another 1000km thus 18000km total.
The route Qantas has stated they will use (at least for part of the year due to weather and winds) actually goes northeast over the Pacific Ocean, over the central US/Canada, over the North Pole, and then into the UK from the north. It doesn’t go over Asia at all.