I have to take issue with Count A Strong's point. A long drive (as long as it is private) leading to a farm over which certain users may have access (ie the postman) is NOT something where the public have access to. The postman, gasman, delivery man has an invitation to drive over that land for the purposes of their job. It is the same as someone walking up a private drive to a house. Therefore, insurance would NOT be needed to such area since the vehicle is not kept on a public road. Supermarkets and car parks attached to areas to which the public are routinely admitted (such as bowling alleys, cinemas, restaurants, stores etc) are different since these are open to the public at large.
On our farm, there were a number of public footpaths and drives. The drives were used for access to the farm only. We drove tractors and vans all over the place and it was perfectly legal (although the fact that I learned to drive in a tractor explains so much)!