I am British citizen, travelling to bkk via guangzhou and planning to stayover 2 nights there.

Ak, you don’t need to apply for any visa because you can enjoy the 72 hour free transit for your first transit in Guangzhou. You can go out and stay in a hotel when using the 72 hour free transit.
As for your second trip, you don’t need it either. But you may need to apply for a temporary stay permit which is free at the airport if you plan to leave the airport.

Many thanks Kathy. Yes, I am planning to spend a night at the hotel outside the airport as I am flying back to London after 18 hrs/ next day. Hope the 72 hrs free transit visa apply for the way back as well???

On the way back, your stopover is only 18 hours, so you can simply use the 24-hour direct transit. Visa is not needed either.

Thank you so much Kathy