Will a booked flight out of HK suffice as proof of onward travel?

Hi,

We are a British couple on a RTW trip who will arrive in Phnom Pehn tomorrow (28th July).

We would like to visit China around the 11th of September for approximately 10 days. We won't have a flight out of China but do have a flight booked out of Hong Kong on Spetember 23rd back to the UK.

Will a book flight out of HK suffice as onward travel?

Thanks,
Alez