I am by no means an expert on insurance but I would suggest that you consult one or more real experts. My best guess is that to get insurance through the dealer will be costly and provide less coverage than if you acquire it independantly. Since you have family in the US, I would start with their agent. Having a US address will probably be much more cost effective than having to insure with your UK address. As a sometimes traveler in Britan, I would also caution you to check out the meaning of every word you have any question about in your coverage. As Churchill once said, the English and the Americans are two peoples separated by a common language. Have a wonderful trip.