For Americans seeking to rent a car in the UK, is an international driver's license required? Sorry, it's a simple question, but it's our first time doing this...
There is no legal requirement for an IDP in the UK.
It's always good to check, so no need to apologise, witness the arguments on the Italy board (where an IDP is required.) Here is the UK govt. website that lists rules for foreign drivers:
http://www.direct.gov.uk/en/Motoring/DriverLicensing/DrivingInGbOnAForeignLicence/DG_4022561
This is only the legal position, rental firms may ask for an IDP anyway above and beyond what the law requires but that is obviously something that varies company to company.