I think it maybe something to do with repairing if accident damaged?That’s interesting, I read online they can be cheaper than a van if re-registered as a camper. Presumably because the assumption is that a camper will only be used occasionally.
As there will be no off the shelf spares available. I have an autotrail scout which is virtually the same premium as my car and reasonable.my friend who has a ducato self build cant even get close premium wise.
Good luck with it anyway and post some progress photos.