Can i go to guangzhou direct from london then go home via hong kong

Yes you can... You can do it in the following way:

- Your flight into Guangzhou has to be a nonstop one or better to say a flight that will not have any additional landing within mainland China or Hong Kong
- You will take a ferry from Guangdong province (where, among others, Guangzhou is) directly to Hong Kong
- You will have to buy your ferry ticket in advance through CKS - Chu Kong passenger transport company website and then print it out. You will have this printout with you as a proof of your transit through Guangdong province. You can depart from Shekou in Shenzhen or from Nansha, just south from Guangzhou or several other eligible departure ports.
- You will have to take exactly that ferry you have bought your ticket for.
- Taking a train is not eligible, but if you want, you can take a direct, nonstop flight from Guangzhou to Hong Kong.

This is the way how you can accomplish your intention by using 144 hour visa free transit scheme.