Some time ago I watched a CAA Examiner brief for overseas Examiners, it raised the issue of insurance so I called the CAA and asked where it could be obtained. Their response was "What Insurance are you talking about?" I then whent round in a big loop and finished up where I started, they had just copied the requirement from the Legislation and didn't have a clue.
I contacted
Max Stein who specialises in Aviation Insurance but he drew a blank as well.
These people might be worth a try
https://www.bearinsurance.co.uk/
or
Besso