In fact it is possible to get "off road" cover, what you need is fire and theft cover only. Third party cover is not needed. The last time I did this with my insurance company it cost me �15 per annum (the car was kept in a garage.)
However you may have difficulty in starting off a new policy on this basis, in my case I was taking my car off the road for a long period following several years of cover with the same insurance company. It may not be as easy for a new customer to start cover in this way. Of course if you do get this kind of cover you must not, under any circumstances take the car on to the road.