Not something I've done myself but as you haven't had a reply yet I offer this -
There are reports that people have insured motorhomes purchased abroad by using the vehicle ID number (VIN). Others have also tried this option but failed to get insurance so it probably comes down to which broker you use. Have a phone around the motorhome insurance specialists (there is a list available from the home page here on MHF) and see if they can help or point you in the right direction.
I'm assuming that you want to purchase in Italy and return to the UK but if you intend on staying in Italy or elsewhere in Europe then have this info ready for when you speak to the insurance broker.