As a visitor, officially you are supposed to have a UK licence and an International licence. I only ever bothered having an IDP for the first few visits. In those days foreigners got stopped far more than they do now. During the stops I encountered I was never asked for my IDP - although I offfered it once and the copper barely looked at it so eventually I stopped getting one.
I have a Thai licence now so don't need anything else but I recently heard of someone who's insurers had refused to pay following an accident - because he didn't have an IDP.
For what it costs and how easy it is to get, I'd get an IDP.