Hi there,
Insurance can and does vary quite alot in price, coverage and entitlements and clauses. What may work with one insurance company for one person/policy may not work with the same insurance company for another person/policy.
Different insurance companies load their risk assessment in different ways. In this case, it's perfectly usual to hear 'sorry, we can't insure you on that car as it's engine size Vs risk assessment (and other factors) means its not insurable with us' - You'll have to try another insurance company and then one that's willing to offer a decent rate to do so.
Having a US license does indeed put drivers in the higher risk catagory - the logic is fairly simple, they perceive the risk as high becuase on paper you haven't got the skills or been tested on the skills for driving within the UK, Get a UK license and you'll have that counted for you in reducing your premium.
As already pointed out, there are other companies willing to add on non UK license holders, just be prepared to pay a significantly higher premium to do so. The risk of course is less if the car in question is one perceived to be a much lower risk. A non UK license holder, on a Jaguar (presumably one of the larger 4 litre and above V8's) and yep, I'm not surprised that a company will say they won't insure you. Hopefully the one mentioned to you in PM will cover you, else phone around. Try Adrian Flux insurance and also try any local insurance brokers, woho may be able to help as well.
Good Luck,
Cheers, DtM! West London & Slough UK!