As Steve says, if there is no policy on the vehicle you won't be able to tax it this is in fact not true BUT very difficult to get around.
From another forum
The 1988 RTA is clear on insurance, and you will find under the VED law that you only have to prove the driver is insured to drive the particular vehicle and it only talks about “The Applicant” not the registered keeper when applying for VED!
There is no law that says the owner and /or registered keeper has to be a driver let alone insure the vehicle, only that a person must not allow a vehicle to be driven on the road with out valid insurance
I put a DLVA adviser into a corner and she agreed that if you have a letter from the Registered Keeper and / or Owner saying you have permission to drive the vehicle and you can give written proof that you are insured to drive it bob's your uncle!
You would have to go to your local VLO to argue the toss and get the VED.
The problem would occur when the vehicle is parked believe it or not, as it would then not be insured!
If it ain't broke, take it apart and see how it works