Many insurance companies will provide cover, but the price and the level of cover depends largely on the level of security at the site where your caravan is stored.
A good starting point would be the CC and the C&CC.
We are insured with the Caravan Club* and have been touring for a few years. We are now considering returning to a seasonal pitch (this year initially) with the possibility of indoor/outdoor storage onsite. We have been in touch with the CC* and have been told that this will not affect our current policy.