You won't get it. Indeed, with insurance charges in this day and age, very few truck rental companies will let a personal hirer take a vehicle abroad at all. Even if you're intending to bring it back. And especially if you're moving out there, beyond the clutches of the British courts.
In my 15 years running various bits of a major UK company, I watched as the number of companies that hire to the public anything above a 3.5 tonne vehicle reduced to none but the small ones. Or the very expensive. And most of those won't let you take them abroad. And none will do one-way.
One of my mates, a bloke who used to work for me in the industry, moved out to Spain last year and even he, with all our connections, couldn't get insurance cover. He finished up doing two trips in a 3.5 tonner.
Sorry to be the bearer of bad tidings, but you'll need to start re-thinking how you do it. There are plenty of companies that will move you, and they will be able to get back loads to keep the costs down (or their profits up!)
I'd love to be proved wrong, but I just can't see that I would be.
Good luck