All you need is a valid US license-- make sure it has at least 90 days left before it expires. Rental cars cost more in Europe; if you can, pre-pay your UK rental in the USA (check with your car rental company on line).

Check to see if your credit card covers rental car insurance in the UK. If so, you may decline the vastly over charged UK insurance. However, make sure you are covered in the UK, and that coverage isn't limited to mainland Europe. (It's the Right Hand Drive thing that may limit your coverage.)

Check with your local office of the AAA for up to the minute details on driving in the UK, including insurance regulations.