Is it possible to do Sydney -> Hong Kong -> Guangzhou (144 transit) - > Hong Kong -> London

Your itinerary is not eligible for 144 hour visa free transit for two reasons:

1. You cannot rely on visa free transit scheme because your segment

Hong Kong - Guangzhou - Hong Kong

Is not a transit, but a return trip. The basic rule of 144 hour visa free transit says that the immediate points before and after mainland Chinese destination, have to be in two different countries or eligible territories (eligible territories: Hong Kong, Taiwan and Macau).

2. You cannot take a train to return to Hong Kong. Train is no longer available as a mean of transport under 144 hour visa free transit.

Your solution for this problem is to return from Guangdong province by ferry, but not to Hong Kong, but to nearby Macau. Like this, you will be formally in transit. I suggest you to do as I have explained and in order to do that, you will buy a ferry ticket from Shekou port in Shenzhen to Macau through TurboJet website, print out that ticket and have it at hand.