The rule for a US licence
GOV.UK confirms that a visitor may drive any small vehicle listed on a full and valid license for 12 months from the date they last entered Great Britain. Because a US license is in English, no IDP or translation is needed.
The 12-month clock restarts each time you re-enter the country, so ordinary tourist visits never approach the limit.
Northern Ireland has its own arrangements; the 12-month visitor rule described here applies to Great Britain.
Live in United Kingdom?
A licence issued in United Kingdom is what you drive on at home - an IDP translates a foreign licence, so it does not apply to you here. If your licence is from somewhere else, check that licence country: the rule depends on where the licence was issued, not only on where you are driving.
At the rental counter
UK rental firms rarely ask for an IDP from US customers, though some require a DVLA check code from UK-licensed drivers, which does not apply to you.
What the permit covers
An International Driving Permit is a translation of your licence into a standard multilingual booklet, issued under the 1949 and 1968 road traffic conventions. It is not a licence, and it is never valid on its own: you carry it with your own licence, and police expect to see both.
It is issued against the vehicle classes already on your home licence, and it never adds one. If your licence does not cover motorcycles, an IDP does not let you ride a scooter in the United Kingdom; if it does not cover a heavier vehicle, the permit will not cover a camper or a minibus either. The permit restates what you already hold, in a form the local authorities can read.
Driving in more than one country?
Check every destination on your trip at once - United Kingdom is already filled in.
Check your tripOther destinations
Informational only, not legal advice. Requirements change and vary by length of stay, rental company, and vehicle class. Always confirm with the destination's embassy or your automobile association before you drive.