Besides the obvious, what are the differences between entering a vehicle "On The Field" or "For Sale"?

Cars and motorcycles On the Field register by class, e.g. MI-01 Mini | Mark I | Mark II and the entrants and spectators vote on their favorite in each class. Information for a "For Sale" vehicle includes the sale price and a picture of the car display on the Vehicles For Sale page. "On the Field" vehicles cannot display any "For Sale" signs.

Is the All British Field Meet dog friendly?

Yes, leashed dogs are permissible. The park rules are that the leash cannot be longer than 8 ft and pets must be under control at all times. Also, if the dog does what a dog does, you are responsible for picking it up.