How can i register a good insurance for my car "bmw z4"?

At first I want to know the best insurance that exists in the market especially the U.S. market and anglaterre,
my car is bmw z4 and as you know it's too expensive, that's why I seek for an effective assurance  in all sides.
Please tell me what do you think about aviva and staveley's insurance.

Many Thanx