The law states that if you have a motorised vehicle (that includes any thing that can propel its self) and you use it in an area that has public access it MUST be insured with at least 3rd party cover, and if you use it on a public highway, open or not, it must be taxed, and you must have an appropriate licence. Your friends who ride their off roaders should be careful, if they get caught, it could still be points on licence and a fine. You may remember several years ago that Tot Rods used to be seen lots at car shows, but they had no insurance or licences, and most show organisers then banned them, with the exception of in a specified enclosed area, a bit like dodgems. the same happened with mini motos and powered scooters.
Sorry to be a killjoy, but if you need your licence, then dont!