I'm actually having the same problem right now. I called Churchill *before* we purchased the new car and got a decent quote. Now that we bought the car and called them back to buy the policy, they're saying that the last guy accidentally entered into the system that I have a UK license and, sorry, they can't insure me. All because the car has a 2.0 litre engine.
This might help some people, though: I called Aviva and they said that not only can they insure me, but if I can prove that I've had five years of no claims that they'll give me the 70% discount, as well (which still only brings my insurance down to £400 a year). I'm still in the process of faxing over all my previous insurance information, but if it works then I'd definitely recommend other people giving it a shot. When I first started calling around for insurance I was told I couldn't get a discount from my driving record in the US, so I don't know how many people know it's possible.