can I get VOA in Guangzhou

Raj, you cannot get VOA in Guangzhou.
You can get VOA in Shenzhen, but you are only allowed to travel around the Shenzhen city. Of course, someone may take Shenzhen VOA to visit Guangzhou secretly, but you are not advised to do that because you may face some punishment once you get caught.
You can fly to Guangzhou from UK, after staying 3 days in Guangzhou, you can fly to a third country or region, and the back to UK. in this case, you can use the 72 hour free transit in Guangzhou to visit there.

Thanks. I'm not actually flying from the uk though. I am in hongkong and also a Hk ID card holder as well as British passport

Will I still get the 72 hour transit stay in Guangzhou if I go from Hong Kong ?

Any other way to get on the spot entry permit for a 2 night visit to Guangzhou trade fair ?

If you fly to Guangzhou from HK and stay there for no more than 72 hours, after that, fly to a third country or region rather than HK, you can enjoy the 72 hour free transit in Guangzhou. In this way, you can visit Guangzhou freely for the trade fair.