If you are traveling to the UK for just a visit or holiday you are legally allowed to drive most vehicles in the UK for a period of up to 12 months on your United States drivers license. You might also want to have in your possession an International Driving Permit (IDP), which you would have to obtain here in the US at AAA.
If you are staying longer than 12 months in the UK you will likely need to get get a British drivers license by taking both a written (theory) test and and a driving (practical) test. Before either of these tests you will have to apply for a provisional licence. You can go to www.dvla.gov.uk for more information.
If you are staying in the British Isles for lengthy period of time and need UK motor insurance get your UK car insurance here.