Flight from Brisbane Australia to London with a 20 Hour stop over in Guangzhou with China Southern

Leo, if you intend to use your Italian passport then you can enter China visa free because Italians do not need a visa to enter China starting from December 1st. If you plan to use Australian passport, your transit will be covered by 24 hour visa free transit and you do not need any kind of visa. However, if traveling on Australian passport, you will have to approach transit counter where you will apply for a temporary entry permit. This application is free of charge and here you will present your flight tickets as well as your hotel booking confirmation. You will be soon granted a permit and you will exit the airport.

If you have bought your journey on a single ticket, then your baggage will be tagged all the way to London. You will just check this detail during your check-in in Australia.

As far as the accommodation is concerned, I am honestly not aware of a hotel located within the premises of Terminal 2 of Guangzhou airport, but I know that there is Pullman Hotel Baiyun Airport within a walking distance from the terminals.