Rarely. Qantas Sydney to Heathrow direct books at about 6% on RTW fare classes. Split it instead: Sydney to Singapore, then Singapore onward to Europe. Both legs book cleanly, and Singapore makes a good 2 to 3 day stopover on the way.
Rarely, and this is the classic "kangaroo route" problem. Qantas flies Sydney to Heathrow direct, but on round-the-world fare classes that routing books at roughly 6%. The flight exists. The seat you need at the fare you need almost never does.
The fix is to break it. Route Sydney to Singapore on Qantas or Singapore Airlines, then Singapore onward to Europe. Both legs book cleanly, and you get a genuine stopover out of it rather than 22 hours in a seat.
Singapore happens to be a good place to spend 2 or 3 days: clean, safe, easy to get around, and worth planning a day around the food.