Travel to Beijing from Hong Kong for 48 hours entry

Hi
I am arriving on a cruise ship into Hong Kong and want to fy onto Beijing to see the main tourist sites for 48 hours/2nights before flying back to Hong Kong for an international flight to the UK. I am a UK national - will I need to get a visa to fly to Beijing and enter mainland China for a tourist visit of only 48 hours? My whole stopover from departing the ship to flying to London is less that 72 hours - so could I use the 72 hour free transit arrangement? Thanks in advance for everyones help Regards Tracy