Travel insurance for private flying

[cal air]

Is anybody able to point me in the right direction regarding which companies in the UK offer medical insurance for private flying in the USA for both pilot and passengers. Myself and a friend are due to go to the states next month for a weeks touring the Southwest but our annual travel insurance only covers flying whilst as a fare paying passenger. Do any companies offer a tailor made insurance policy which they can get to complement our annual travel insurance policy? Also whilst renting an aircraft from an American flying club does the renters insurance cover you even if you are a foreign national. I just don't fancy getting caught up in the insurance small print just in case we have any mishaps while we are away next month as I am sure you all know what the Americans charge for medical bills and liability claims.
Any help greatly appreciated.
Cal Air