Can I get 72 hour visa from Hong Kong after coming from London

I am a UK citizen flying to Hong Kong to visit my son. We will then be spending a few days together in Malaysia before flying back to Hong Kong. Before returning back from Hong Kong to London I have a few days and would like to fly to Xian for a couple of nights. Would I be able to do this on a 72 hour free transit and if so how do I apply for it?