If you are basing yourself in the USA then you stop paying National Insurance Contributions and the UK stops providing your care. If you go you are no longer our responsibility.
The system does record your contributions and the point will come when the UK is no longer responsible for you in any way. Bon voyage, enjoy the trip - do remember to make your own pension arrangements, health care provision and tell HMRC that you have gone.
Goodbye.
You can buy annual policies but I don't think many of them will cover you for extended stays in the U.S. and they might be quite expensive although extremely essential. If you have an American Express card try them. They certainly do travel insurance. If not just contact the major insurance companies and see what they come up with
If you're living in America, its no longer travel insurance. You need to find a health insurance provider in America.
You can buy travel medical insurance from any travel agency or your airline, but TMI doesn't pay for ongoing medical issues.
Check these two sites for info and options. AAA will apparently insure UK citizens visiting the USA for extended periods.
http://travel.state.gov/travel/cis_pa_tw...
https://purchase.imglobal.com/Quote/patr...