Siem Reap - Shanghai via Guangzhou - London

Hey!

We fly from Siem Reap to Shanghai on a flight that connects in Guangzhou. Then we fly on to London from Shanghai.

Literally...
1. CSA3054 Siem Reap to Guangzhou
(2 hours later but on the same ticket)
2. CSA3581 Guangzhou to Shanghai
(less than 72 hours later)
3. BA168 Shanghai to London

Will I have to get a visa to transfer in Guangzhou? Will I be able to get the 72 hour free transit in Guangzhou and then exit in Shanghai?

Thanks