Seoul - Shanghai - London

Yes, you can. If your return trip:

Seoul - Shanghai - London

Does not have any additional landing of any kind between the specified points, you are definitely eligible for 144 hour visa free transit in wider Shanghai area and you do not need any kind of visa. Have a nice journey.

Thank you! I should have said I am flying London to Seoul for 2 days, then Seoul to Shanghai for 3 days, then Shanghai to London, does it make a difference that my starting point is the same as my end point?

Actually, the segment that is of interest to Chinese authorities is only the immediate points before and after your mainland Chinese destination. In other words, your immediate points before and after your mainland Chinese destination are Seoul and London and that is eligible because the basic rule of 144 hour visa free transit clearly states that the immediate points before and after a mainland Chinese destination have to be in two different countries or eligible territories (eligible territories are Hong Kong, Taiwan and Macau). In other words, you are good to go and you do not need a visa.