Sometimes it's cheaper if you join a club. We get a good saving on our classic car policy because we're in the Humber Club and National Street Cars Club for our collection.
OH has insured our 1992 T25 Multivan on our classic car policy with Footman James for not a lot. We do have 4 other vehicles on the policy and it add the T25 cost us under £100.
Theft of T2s is not uncommon due to high price of them now, vehicles are targeted & thefts organised. I gave up with them when they stopped being campers & turned into classic vehicles. So not suprising insurers are insisting on security nowadays.
I would think if you insured without agreed value it would be easier but doing that you risk losing an awful lot of money. You might get better advice on justkampers forum. Have you joined owners club? £25 but might be worth it.
Adrain Flux may be fine until you have to claim! I'm with Classic Line and I'm sure they are fine with non-garaged vehicles. A lot of folk are with RH Insurance, but you'll need to check if your camper van needs to be garaged.
Don't forget to add layers of security, they just drag it off your driveway.