Yes, in a true plane-free world you'd probably get from London to Sydney by taking a train to Shanghai or Bangkok or something and then catching a ship to take you the rest of the way.

Or with a bit of high-speed rail development, get a train to India, a boat across the Indian Ocean to Perth, and then a two-day high-speed train across Australia.

This actually sounds like fun the first time, but if you had to do it regularly you'd be missing the 23 hour plane ride pretty quick.