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Americans are legally allowed to drive in the U.K. for a period of up to twelve months on their U.S. driving license. The 12 month period commences upon the first day of your residency in the U.K. If you will be resident longer than 12 months, you will get to experience the joy of taking both a written (called theory in the UK) and driving (called practical in the UK). Additionally, you must apply for a provisional licence before taking either of these tests. The tests are typically spaced out, too. You cannot take both tests on the same day. See thedriving requirements for non-EU citizens on the DVLA site to obtain full details.

http://www.americanexpats.co.uk/driving.htm