I've been researching this recently and while I've not contacted them yet in my situation (I have an EU license now, no longer a US one) this seems promising from Aviva:
----------------------
You asked:
What if I have earned No Claim Discount on motor insurance overseas?
Our answer:
We will automatically accept No Claim Discount (NCD) if it was earned from any of these countries, if you earned NCD elsewhere please call us on 0800 092 3615.
European Union (EU) and European Economic Area (EEA)
Austria, Belgium, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of Cyprus (South), Republic of Ireland, Romania, San Marino, Slovakia, Slovenia, Spain, Sweden
Others
Andorra, Australia, Barbados, British Virgin Islands, Canada, Gibraltar, Falkland Islands, Faroe Islands, Hong Kong, Japan, Monaco, New Zealand, Republic of Korea (South), Singapore, South Africa, Switzerland, United States of America, Zimbabwe
We may ask for proof of your NCD which should:
> Be in English
> Be on official company headed paper
> Be in the main driver’s name
> Show the vehicle details
> Confirm the number of years the NCD was built up over
> Show the date the policy expired
Please note that your NCD will be valid for 2 years after the expiration of your last policy, provided it is not being used on another policy.