Indirect London - Beijing - Hongkong

Hi , I have ticket for Beijing, and then I will go Hongkong , Macau and Bangkok.
I wanted stay 6 days in Beijing, now i found out that i need visa and is cost £190 (apply by post).
I want change my hongkong's ticket date to be able get free transit
I will have my hongkong ticket in hand but without confirmed seat !!!
I'm British. Can i get 72 hours free Transit in Beijing Airport ?
Thank you