You do not need a tourist visa. Come without a visa and your will get stamped in for 30 days. You can extend that for an additional 30 days at the Immigration office for 1,900 Thai Baht.
John, 1. Thai Embassies are not issuers of Thai traffic law. Information on traffic rules published on their sites are just unsubstantiated information. 2. It is international conventions on driving in foreign countries that the Thai (and many other Governments) signed up to. I understand that there are 2 of these conventions (the second updated the first). Nowhere in either convention does it even mention IDP's, let alone specify that they are required.
Crap. The Royal Thai Embassy in London is not an authority for Thai driving laws. The authority is the international convention of driving signed up to by most countries in the world, including Thailand. The convention allows tourist to drive on their national licence in Thailand. If their national licence is not in English, then an English translation is required (obviously, an IDP meets this requirement but an IDP is not required if the licence is in English).