Dear Fergus,
Good morning, this is Kristy from Top China Travel.
Thanks for contacting Top China Travel! We have replied to your tour enquiry, please check your e-mailbox.
For your question, British passport holders can enjoy China 72 hours visa free policy in one of Chinese authorized cities exit and entry related regulation. You can walk around the authorized city during this 72 hours.
One thing should be attention is you can stay only one of the authorized cities. That means if your international flight arrival Beijing, you should hold Beijing leaving ticket. If arrival Shanghai, Shanghai leaving ticket only.
Any further request or question, please feel free to contact with us.
Regards
Kristy