Most call centres will operate a "non advice" policy and cant advice because there not qualified. Your van would be fine on most private car policies as long as its use is not commercial.
Really I would go back to admiral and speak to someone else " you are seriously telling me I cannot change the vehicle insured to a small van?". It will either be that they dont cover vans at all (unusual) or they dont want to cover you in the van (occupation or something).
Get someone to explain why you cannot be covered be assertive and get a reason.
If they still wont help take out another policy for your van then apply the NCD from your existing policy when the first policy expires.
You shouldnt sell your car without informing the ins company its in the contract but you can weigh up that risk for yourself.
Overall though you should probably check you insurance prior to buying a new vehicle or accept the consequences.