If you want coverage for a cruising area outside the 12-mile limit, why don't you just ask your insurance company for it?
If they have a problem with agreeing it, do as JK says and vote with your feet.
We pay very little extra for full Brest-Elbe coverage. One insurance company offered it for nothing last year.
You should be able to 'phone up your insurance broker and add risks to your policy as and when required. If they don't offer this flexibility, don't offer them your business.
We us Colin Curtis Marine in Plymouth and every risk we have ever asked for has been added to the policy at very modest cost. http://www.curtismarine.co.uk
Euromarine are also very good at tailoring a policy to your requirements http://www.euromarine-ltd.com
and Porthcawl offer a good service to RIB owners http://www.porthcawl-insurance.co.uk