You may not need an International Driving Permit (IDP) if your driver's licence is issued in Canada, Australia, New Zealand or USA. Only those with a licence that is not printed fully in English will need an IDP. Remember that if you are asked to travel in the UK with an IDP, you will still need to present your original driver's licence when you pick up your rental car. The International Driving Permit serves as a translation only, and will not be accepted if it is not accompanied by your licence.
Usually, an International Driving Permit is issued by monitoring organisations in the country in which you are licenced to drive. So if you are a foreign visitor to the UK, you cannot be issued an IDP from the UK. You will need to apply for your permit from your country of residence. Note that an IDP is only valid for one year from the date of issue and it cannot be renewed. You will need to apply for a new one if yours has expired.
Read more about Licence Requirements for renting a car in the UK.