I have booked a return flight from London to Phuket with Shenzhen Airlines.

My flight stops in SHENZHEN, CN (BAO AN INT) for a few hours on both the outward and return journey. Do I require entry permit for this? I have read different things online and had different answers from Visa arrangement companies, so I wondered if anybody could please clarify? I hold a British Passport. Do I only need entry permit if I leave the airport (which I do not intend to do)? Thanks in advance.