If it's not on a public highway (thus on private ground) then you may declare a SORN otherwise it must be taxed and anyone may pay the tax as long as they have the insurance cert and MOT (if applicable).
Unless the car is inaccessible to the public you should speak to the insurance company about insuring the vehicle against theft and damage by vandalism or attempted theft but I would advise you get someone to immobilise the vehicle anyway.