Insurance for an American using a car owned by British subjects

For 3 weeks car insurance cover, this would fall under the short term car insurance policy (1-28 days) and the problem is that all insurers which offer temp cover do not accept licences from USA/Canada/Australia. This might be because these countries do not have similar rules as European countries which consequently make it harder to process a claim. Due to the nature this type of car insurance which makes it rather short in duration but carrying the same or more risk as an annual policy, it is restricted by many insurers to avoid complications.

Are you not able to extend your car insurance cover in the US to drive here in the UK although you will be driving someone else's car?