I think you are in luck because you are buying from a family friend. Just keep It registered in his name with you named as a driver on his insurance, with a European driving clause in the policy. When you have put it on the ship to the USA then do the registration transfer. You can do the sales contract at any time if you trust each other.
I have done this twice importing cars from the UK to continental Europe. It is legal and allows time to go through the process of getting it registered in another country.
It is almost impossible to get a UK insurance company to insure you if you don't have a UK address and a record of being insured there. But I have encountered no difficulties, as holder of an EU license, of being named on UK insurance policies as an approved driver. Hope this helps and that you have a successful ownership.
