Are you planning to stay on a site all the time, or would you be driving your van to and from work etc...?
There is a filter for open all year round camp sites that one can apply when searching for sites using the UKCS search facility.
You can forget C&CC club sites as they only allow a maximum of 14 days on a pitch. Not sure if they would allow one to stay longer by moving to a different pitch though.
Here is the search result threads on living in a caravan for the past 2 years in the Caravan and Caravanning section of this board, worth a look as some of the issues considered also applies to motorhomes and campervans: