How have you converted the van Polly, have you made structural changes i.e. fixed furniture, ehu, gas supply, ect. or have you just thrown a mattress and a bit of camping gear.
If the changes are permanent then you will have to reregister as camper if you haven't done anything permanent it can stay as a van.
Is it worth trying to get a few quid off the cost of the crossing and risking an Insurance claim that could possibly go belly up, it will cost you a damn site more.