The International Driver's Licence, is really just a supplement to your official licence, so if you don't have a UK or a US licence, you won't be able to get or use an International licence (i.e. it is not valid for use abroad without the UK one to accompany it).
Thing is, because the international licence doesn't really mean all that much in the grand scheme of things, a lot of people don't bother to get one because the UK licence alone is sufficient for driving in the US.