Well, you say he 'is allowed' - given that the Cunard ships are registered in Bermuda, I don't really see how that can sit with the below:
I'm slightly miffed they are allowed to fly a UK red ensign as it is.
British merchant ships and fishing vessels are allowed to wear the plain Blue Ensign under the authority of a special Warrant, subject to certain conditions being fulfilled, and which are outlined below.
Vessels registered on the British Registry of Shipping may wear a plain Blue Ensign providing the master or skipper is in possession of a warrant issued by the Director of Naval Reserves under the authority of the Secretary of State for Defence, and the additional conditions outlined below are fulfilled. The Blue Ensign is to be struck if the officer to whom the warrant was issued relinquishes command, or if the ship or vessel passes into foreign ownership and ceases to be a British ship as defined by MSA 95.