According to the AA website and International Driving Permit applications website,they are fining for not having a IDP.
I’ve just renewed mine. Cost £5.50 from the Post Office (+photo), so its hardly expensive. I must say, however, that in nearly 30years, i’ve only ever had to show it once.
theaa.com/motoring_advice/ov … untry.html
As the AA sell the permits, it may be a trick to relieve you of your money too. 
Although they are also available from the Post Office nowadays