I know Patrick Barber brought his Spitfire and also a GT6 over to the UK for the RBRR, but I'm pretty sure he used Hagerty at that time. I also know another guy that took his TR3 to the EU for an event and he also used Hagerty, but it appears they no longer offer insurance in the UK/EU for a car on US plates.
I have contacted Cherished, Peter Best and Adrian Flux with no response yet. Classic line said no as did Hagerty UK. I can get liability only from Tourinsure, but that really isn't a good option.