Car tires don't provide insulation protection either. Air is an insulator, though not as good as rubber, but instead of a couple of inches of rubber, there are miles of air which the lightning is travelling through. Any insulator can be overcome with sufficient voltage. What protects you in a car is called a 'farraday cage'.
Nothing on the bike will protect you; however, since it is a charity ride I would expect the organizers to postpone if there is any real danger.