Your'e right, it's a bit confusing, are you talking about personal cover without an MV exhibit?
I would have thought most classic car/military vehicle policies included show cover for you whilst driving you MV as this is what most owners do with their vehicles.
Have a close look at the conditions of your policy, it may be that you will find you are covered, it is not unheard of that the person who answers the phone does not know much at all, or is even interested!
Even if you just had an ordinary third party cover only, I'm pretty sure you would have the necessary public liability to drive onto a show ground and park on display, after all that's no different from parking in the car park, arena displays might be another matter though.
Interesting and important stuff this, do show organizers actually know what is needed or do they just put that clause into the form to cover themselves?