72 hours VISA WAIVER British passport

Hi I wish to arrive in Guanzhou via Hong Kong by high speed train after a cruise on 1st Jan 2020 at around 09:00am and have a flight to Nairobi (Kenya) on 4th Jan 2020 at 04:30am. I have a British passport. Would I be eligible for the 72 hour transit visa? If yes, since the timing is perfect would I be permitted to fly to Harbin for the ice festival for one night and return to Guanzhou within that time?
Thank you!