240 hour transit visa

Yes, your itinerary is eligible for 240 hour visa free transit. I recommend you to use correcet terminology because this is not 240 hour transit visa, but 240 hour visa free transit or in other words, there is no visa of any kind at all.

As far as your segment from Guangzhou to Hong Kong is concerned you can travel by plane, ferry or train. Regardless wich of the mentioned options you choose, you will choose to use, you will have to buy a ticket even before your departure from London because this will be a proof of your transit intentions. Also, ferry or train tickets have to be presented as printouts. So, if you buy your ferry or train ticket, you will have to print them out and have at hand because you will have to present them in both London and upon landing in Shanghai. You will perform your immigration formalities in Shanghai because it will be your first landing point in China.

When you land in Shanghai the very first thing you will do is to approach 24/240 hour visa free transit counter where you will apply for a temporary entry permit by presenting your flight and other tickets and a hotel booking for at least the first overnight. When your permit is granted you will head to the immigration and then to your flight for Chongqing. Please, have in mind that you will really have to take exactly that ferry/train/plane from Guangzhou to Hong Kong that you have bough for. Have a nice journey.