To drive through Iran you'll need a Carnet de Passage. This is a customs document allowing you to temporarily import your vehicle. They are generally sold through the automobile association of the country where your car is from. Information can be found here on where to apply for your carnet, for the UK it will direct you here.
Past ralliers will tell you conflicting advice about the best way to sort out your carnet. The past few years there have been a few teams who've sorted out their Carnet at the border with either a pre-arranged fixer or taking a chance on their own. These teams have reported that it can be much cheaper than sorting it out in Europe.