Plymouth British Fireworks 8th-9th Aug

Plymouth Sound site is a natural choice overlooking the Sound.

Riverside commercial site at Marsh Mills could be worth considering as it’s adjacent to the park and ride which usually operates during the championship.

http://www.riversidecaravanpark.com

Those are good suggestions from TW, but if they are full then Compton Park at Callington is another possibility which may have room as it's inland - about 20 minutes drive. If you like the style of Club site hard standings you will like this one. 

If the Plymouth Sound site is full alternatives are the CL next to it and the rally which takes place in the adjoining field. All in Down Thomas, not far from Jennycliff for good views over Plymouth. I'm sorry I can't remember which centre is running the rally, a call to the club might help as the general on line rally search is hopeless unless done via each centre.