Normal car insurance companies struggle with the unique requirements of campervans. There are a number of specialist insurers that provide good cover for campervans, although these may have restrictions on driver age, vehicle value and conversion type.
Despite your UK insurance being valid anywhere in the EU, most insurers will limit the time you can spend there on any one trip. Some will also limit you to just EU countries - but this now encompases many more places and will be expanding again in 2007 when Croatia, Bulgaria and Romania join.
Before contacting a broker make sure you have all relevant details to hand. They will want to know much more that your typical car insurance broker, particularly about the conversion. Specify what you require for foreign cover and specify the countries that you want to travel to. Usually brokers will offer you the cheapest cover, but this generall doesn't give you everything you need so shop around and be willing to pay a little more.
Recommended insurers are [Adrian Flux] who deal mainly with small vans and will insure pretty much any campervan. The [Caravan Club] is more suitable for larger vehicles and offers good deals to members - the money saved on insurance is frequently more than memembership costs.