Last week I put in an official complaint to Nationwide about the restrictive definition of a Cruise on page 12 of the Nationwide Travel Insurance policy document.
On Thursday I received a telephone call from a representative of UK Travel Insurance, the Underwriters, who have explained the situation.
As of now Nationwide are not going to change the definition in the policy document. However, they will provide basic cruise cover under the policy but will no longer offer the optional extended cover that includes things like missed ports and cabin confinement to companies who are not Cruise Line International Association members.
This is the official line from Nationwide.