Hi Espee
I would recommend Aviva, they were very good many years ago (year 2000) when DH and I went to the US.
Thankfully we had taken out a medical Insurance, as the day after we landed in New York, DH had a heart attack in the street. Fortunately we got to hospital quickly by taxi, with him lying down in the back.
The medics were great and a few days later he had a cardiac stent fitted.
Everything turned out ok and we were able to return on our planned flight.
It was the best £39.00 we ever spent, God knows what the hospital bill came to, we never did get to find out.
The insurance company even called our GP to check for previous history before paying.
I wish you a great trip, just make sure you get good medical cover.