Also looking back a few years ... but I had no problem adding Lisa onto my motor insurance while we were courting and before she lived here, and without an International driver's license. Her Florida one was enough. I understand that most (but not all) insurers will do this.
If you want to get insurance in your own right, in a vehicle that you own, in the UK without a UK license, you may find it harder but it can be done. As I recall we used Norwich Union ... but bear in mind that Norwich Union Direct and Norwich Union are different parts of the same organisaition and you may find that "Direct" refuse you but the main company, via an agent, accept you.
You could always hire a car and leave the headache to the hirers who I'm SURE will be set up to insure tourists from the USA