I thought an update might be of interest. I've rung up many more companies. The best I got was Adrian Flux. After being on hold for 10 mins and being passed round the houses I was told if it was a wandering insurance to cover driving his own car anywhere in Europe then they could help. Otherwise no one will let me insure my brother on my car.
I asked the person on the phone what if I want to insure a car I keep overseas. He didn't know what the situation would be then so I came at it from a different angle and said I want to insure myself on my brothers car in the USA. His answer was "get a hire car". Very helpful...