Hong Kong to Shanghai to London

Hello i have recently been staying with my friend in China for 10 days. I had a single entry visa into China. I am now in Hong Kong so technically i have left china and cannot return to the mainland however i have a flight booked back home to London from Shanghai. I will be in Shanghai for 2 days staying in a hostel, will i be able to do this on the 72 hour entry permit free service? If so where can i apply for it? Will there be somewhere in Hong Kong airport? Thanks