Just a quick thought there are a lot of classic car events that go all around europe and they must have some sort of insurance system that allows them to do the runs.
Just another possability the company that offers BX club members insurace discounts why not contact them and see if they could be a named sponsor by providing the insurance for the event as they would get advertising on each and every car. (And as a group policy for the event they would be able to save money) Worth a try!
A few years ago my brother competed in the italian job run which is also a charity event and i know the organisers of that event.
I will try and get in touch with them and find out any useful information.
I will keep you posted.
Regards Kermit