Travelling from UK to Shanghai for cruise to Japan and back to Shanghai

Your entire itinerary is definitely and undeniably eligible for 144 hour visa free transit in Shanghai twice. It is important to understand that the Chinese authorities will not consider your cruise as a return trip to Japan because the segment they are looking for consists only on the immediate points before and after a mainland China stop and those immediate stops have to be in two different countries or eligible territories (eligible territories: Hong Kong, Taiwan and Macau).

In both cases, your immediate points before and after Shanghai are located in two different countries and that is all that matters. You will hardly get any definite answer or better to say a confirmation for what I am writing here about because all their rules and conditions are loaded into Timatic, IATA published tool, that is used by all border authorities of the entire world. UK authorities will use it as well as Chinese and Japanese. You can access Timatic if you check at the website of United Airlines that still offers complimentary public access to this ultimate tool. Beside Timatic you can access the website of the Chinese National Immigration Administration that is in charge of this subject and there you will be also able to see that you are eligible for your planned journey absolutely visa free.