Hey Dave,
It was a case of knowing someone well to get the van and take it abroad for the last few years. Our normal guy's business is closed now and this year we had to rent from Avis. Getting Insurance is a bloody minefield and we had to call in a few favours to be able to leave the country with the van. I can't even remember what story was concocted but we managed to get it for the trip in the end thank God. As the old van was a crew cab, the Avis Rental was only a three seater and we had to book an extra car onto the ferries less than 48 hours before we left.
If at all possible, try to borrow one from someone you may know, maybe give them your car for the week or so while you're away. If that person's van is commercially insured, the easiest way to get around the insurance is to get yourself added as a named driver on the van owners insurance. This will cost between 50 and 70 quid but it's worth it for having all the extra space.