If you have specific aircraft (by registration) that you wish to ‘target’ an invite to your event, then the CAA G-INFO website provides a paper-mail address. So drop the owner a letter outlining why he is invited, why his aircraft fits the event theme and the advantages to him of attending (free landing fees, discounted fuel, free lunch etc). Responses will vary from enthusiasm to consigned to the bin, but that’s the trials of being an event organiser.
Don’t overlook the aircraft already at Blackbushe, while the owners may not need to fly-in, a taxi across to the ‘public’ fence-line provides interest and makes the event look larger than the number of landing aircraft permitted!