With a US driving licence, you will find it hard to get temporary car insurance I'm afraid. Actually there's no insurers at the moment which will insure you at all. The international driving licence does not make a difference unless you're taking an annual car insurance policy.
Will your car insurance company in the US not extend its cover to the UK? It might be worth looking into that because here in the UK, you will not be able to drive unless you're insured to do so and you won't get cover with your current american licence.
