I coordinated the East Anglia Off Road show at Bircham Newton for Breckland Land Rover Club raising funds for the East Anglia Air Ambulance for a while (raised £9000 one year) so believe me you need massive support from the members of an organisation like BLRC to make these things work. Health and safety issues alone are a minefield, with risk assessments, fire safety, planning, licensing, insurance, let alone the need for a road traffic act exemption certificate. Charities like the air ambulance are 100% happy to take the money raised but offer very little support in the planning and running of the event. Licensed sites like Manby will charge a heft fee per vehicle making it difficult to raise much at all.
Don't get me wrong I wish you well, but think small at first. Perhaps arrange a site discount for a few mud clubbers to meet for a day offroading and raise a bit of cash for the air ambulance by agreeing to pay full whack and pass on the discount, or run a simple competition between the attendees.
Best of luck, and merry Christmas to all.
Mike