That makes more sense! Are you travelling by chance from HKG ...?
As long as your trip is XXX - CTU - LHR, where XXX is somewhere outside China, then with a UK passport you are unequivocally good to go regarding the transit-without-visa facility for up to 72 hours (counted at CTU from midnight on the day of arrival).
The current Timatic details, if you need them, are in the wiki in the "China-24-72-144-hour-transit-without-visa-twov-rules-master-thread" over in the Destinations/China forum (sorry can't post a link at the moment due to technical VPN-related difficulties ...)