Shenzhen to Hong Kong Internationa Airport

Hi I am travelling to China in March arriving at Shanghai. I have a two year multiple entry Chinese visa which expires in July 2018 and I am a British Passport holder. I am visiting a Chinese friend in Shenzhen at the end of my holiday and therefore I have booked my return flight to London from HKIA. Will I encounter any problems in coming from the Chinese mainland to Hong Kong. Will their be a requirement for any other visa or documentation.? I will probably be staying close too or in the Luoho district of Shenzhen. Which is the best method of travelling to HKIA?? My flight departs late evening on Tuesday 4th (which I think is a Chinese holiday). Many thanks