Entry to mainland via port/land car rather than plane , qualify?

Unfortunately, this is not possible because only the entry by plane to Shenzhen, Guangzhou and Jieyang Chaoshan airports is an eligible way how to enter Guangdong province. You can exit the province toward Hong Kong or Macau by taking, for example, a ferry, but the entry has to be by plane. So now your option is to take a flight from Hong Kong to Guangzhou and then to fly out toward Europe by a direct, nonstop flight. As far as I know, there are no direct flights from neither Guangzhou or Shenzhen to Berlin, but you have direct flights from both of them to Frankfurt.

Another option would be to really take a car or whatever to Lo Wu (Luohu) or Huanggang border crossing and to apply there for an ordinary Chinese, single entry visa that is valid for the entire country. The price of this visa for UK citizens is 626 CNY and when the visa is granted, you can travel to the entire country and you can also return to Hong Kong or fly out from any mainland Chinese airport toward Europe. Please, just do not confuse this ordinary, single entry visa with Shenzhen 5 day visa on arrival that is noticeably cheaper, but it also limits your movements to Shenzhen city limits only.