Is 72 hour visa free transit applicable

Travelling with a UK passport ... journey as follows

London Heathrow to Hong Kong (British Airways) 66 hours in Honk Kong, Hong Kong to to PEK (Cathhay Dragon) 72 hrs 55mins flying on to London Heathrow (British Airways)

Can i apply for 72 hour free transit or do i need full entry permit for the stay in China in order to be able to leave the airport??

thank you