I did a quick search on the internet, I found the following information:
You will need to provide an International Driving Permit if your own driver's licence is not printed fully in English. In most those with a driver's licence issued from the USA, Canada, Australia or New Zealand do not require an IDP to be able to rent a car in the UK. However, keep in mind that some car rental companies may require one if you are planning to drive across the channel, crossing by either Le Shuttle or a ferry. Make sure to call and confirm this with your car rental provider.
A United States citizen who normally resides in the United States and is visiting or temporarily residing in the United Kingdom may drive by virtue of his/her valid American license for a period of one year from the date of his/her last entry visa.
Again, found on the Internet, so take with a grain of salt..... you do not need to an IDP if you drive in the UK for visits up to 30 days. For visits longer than 30 days you need to have an International Drivers Permit under the 1949 Convention. I do not know whether Thailand issues an International Drivers Permit under the 1949 Convention.
Other posters have provided you where to obtain an International Drivers Permit which is Chonburi Department of Land Transport. If I were you I would go and get an International Drivers Permit and be better safe than sorry.
I would not take any answer here as definitive and instead go to the UK Immigration website and search for the authoritative answer.