144 hour visa

"144 hour visa" doesn't exist. There is 144 hours transit without visa, but you are not in transit in China. You must either get a visa or change your flights so that you are in transit.

To overcome the problem, I would suggest you to travel from Guangdong province to Macau and then from Macau to Hong Kong. So, your itinerary would be London - Hong Kong - Guangdong (this portion has to be performed by plane, so fly Hong Kong to Guangzhou) - Macau (take a ferry from Shekou Shenzhen port to Macau - Hong Kong (by ferry from Macau). You have to book your flight Hong Kong - Guangzhou in advance and also the ferry transfer on TurboJet website. You need these tickets even before making a check-in for your flight from Hong Kong to mainland China.