With regard to the private land thing some of you have mentioned, the law was ammended in April 2000 and since then you must be insured to use your vehicle on the road or other public place
The following 3 paragraphs are taken from the "Cop Shop" article of another club I belong to.
Other public place is not defined, but is generally taken to be any place to which the public have access whether on payment or otherwise.
In short, this may effect such people as learner drivers starting off in car parks, or vehicles used for games, or displays at Club events. The scope of these regulations is huge and virtually each occasion a motor vehicle is used anywhere where the public have access means that it must be insured and MOT Tested.
A driver of a motor vehicle in an other public place is required to produce an insurance certificate and test certificate to a constable only where the constable believes there to have been an accident as defined by the act. Similarly, the regulations do not provide a power for a constable to require a driver of a motor vehicle in the extended other public place to produce a certificate of insurance and test certificate unless the constable suspects that the driver has been involved in an accident.
I know this only deals with the insurance aspect of driving in a car park, but presumably the law does apply to driving in car parks otherwise a policeman wouldnt need the power to get a driver to produce certs. if you have an accident in one.