That sounds odd. An IDP is normally for when you have a licence in a different language, to allow it to be interpreted in the country you're visiting. The rule for the UK is that you can drive on a foreign licence for up to a year. I'm assuming your licence isn't in broad Strine?
A number of car hire companies insist on additional ID, including proof of address. Generally, I only come across this when hiring a Luton van or the like, but they may be stricter with overseas visitors.
Do you have a link to the site?