From the official website:
"The main ground event site site is located on Marine Parade West (///: choppy.smiled.gazes) However, there is adequate viewing available along the seafront from the East side of Clacton Pier (///:forgiven.verdict.succumbs – towards Holland-on-Sea) to Clacton Martello Beach (///: starlight.repeated.protests) including our upper and lower promenades and West Greensward.
https://clactonairshow.com/faqs/
That basically mean that anywhere along the seafront will be suitable for viewing. My own instinct (as someone who knows Clacton well) would be to avoid the crowds around the pier and to head for the grassed area alongside Marine Parade West (above the promenade), roughly opposite Nelson Road and St Vincent Road. (Take some camping chairs). There's likely to be a bit of space there and you're fairly close to the loos and beachside cafés. (For something more substantial, you've got the Toby Carvery alongside the promenade. There's also the cosy Seaspan pub nearby, just up Wash Lane).
Sadly though, the weather forecast for Clacton on Thursday is for light rain for much of the day, with thundery showers predicted for Friday.