Very few UK based insurers are able to provide cover in that area. I suggest that you contact Admiral Marine, telephone +44 (0)1722 416106 or www.admiralyacht.com
I used Mariners General via their Florida office in Bradenton.
Sound established company and very helpful when I needed to make a claim.
They were very flexible in their approach
(I had british registered vessel moving from different cruising areas around the USA and Bahamas)
You could try Pantaneus. They are currently insuring me from 4 degrees to 50 degrees north, atlantic and caribbean waters, plus the med, and if you have a claim they pay up.