According to the regulations of China's 240-hour visa-free transit policy, your current itinerary does not meet the application requirements. The core requirement of this policy is that the immediate points of entry and exit to/from Chinese mainland must belong to different countries or specific administrative regions (Hong Kong, Macao, and Taiwan). In your original itinerary, both the departure and arrival points for entering and exiting Chinese mainland are within the United Kingdom, which falls under a single sovereign state. I recommend considering optimizing your itinerary structure, such as adding France as a transit node, to form a route like "London → Paris → Shanghai → Beijing → Xi'an → London." After adjustment, the immediate point before entering Chinese mainland (France) and the destination after exiting (United Kingdom) will belong to different sovereign states, thus meeting the policy requirements. If you hold non-refundable tickets, you will need to apply for an L-category tourist visa in advance from the Chinese embassy or consulate abroad. Regardless of the option you choose, I wish you a safe and pleasant journey.