Ahhh, I think there was some confusion. When you said National Motor Museum I took that to be Beaulieu. I presume you actually meant the British Motor Museum located in Gaydon. ie
https://www.britishmotormuseum.co.uk/ not
https://www.beaulieu.co.uk/events/
Like Beaulieu, Gaydon do hold events and I'm sure if you were to approach them they would oblige, providing that you could provide 'evidence' that you could get the sort of numbers they need.
Indeed back in 2016 MITCAR was held there
https://www.aroc-uk.com/eastmids/mitcar2016rep.html
(MITCAR is always held in a different location each year, so won't be back at Gaydon)
Out of interest what is your business? Are you in a position to actually sponsor an event (indeed are you in communication with other businesses who might combine sponsorship). If you could provide Gaydon with a plan for sponsorship/promotion that would help.
Am off to Haynes soon. Looks like it's going to be a nice day.