I would suspect that if a caravan is stored and used only on the Continent then it will probably be classed as a resident vehicle of the country concerned and need local registration before a local Company will give insurance. Which means you need an address there. No UK Company to my knowledge will give insurance to something based in another country. Ironically our European cousins get unlimited cover in all other EU countries for as long as they like, because they don't have the watery bit in the middle, or as usual the Brits get short straw.