Flying from Beijing To Hong Kong then back to Shanghair to catch flight to UK on the same say

Hi I have a one entry 30 day visa for china which I have used on my arrival in Shanghai. I have a flight booked depart from Shanghai on the 30th day of my entry permit.

Before I leave, I want to fly to Hong Kong. Will l be able to fly back to Shanghai after visiting Hong Kong and use the 72 hour waiver to catch my flight back to the UK? I can't use my entry permit as it has already been stamped. I don't plan on leaving the airport and it will just be just like catching a connecting flight.