I don't know whether it will be of any help but I have always used Coleman Marine in Poole who were a specialist marine insurers, underwritten at LLoyds. When I was building the GH and doing a lot of repairs on old ones, they were very positive and did a special GH policy. For the older boats they would often waive the survey requirement if I had cast an eye over the boat and said it was sound. They also accepted a simple "general condition" statement from me as a boat builder, in lieu of a surveyors full report, on several other types of boat.
However, they are now much bigger than they were and things might have changed - they are now Sunseekers insurer's. But it might be worth having a word with them and saying that you know that in the past they have accepted a statement of condidtion from a bona-fide boatbuilder, and, if they will, find yourself a local small builder who would be happy to earn a few quid. If not, I'm sure they would accept a decent home build if you got a proper survey report. They have done so often in the past.
E Coleman & Co Ltd.