May be a little confusion here.

British Maritime Ensigns

The Red Ensign Slang "The Red Duster (Red Flag with the Union Flag in the upper home quarter) - shall be flown by a Registered British Vessel when leaving or Entering Port

The Blue Ensign ( Mid blue with the Union Flag in the upper home quarter) - Worn instead of the Red Ensign by Naval Auxillary Vessels ( Supply ships etc )

British Registered Ships where a substantial proportion of the officers are Members of the Royal Navy reserve may apply for a Warrant to fly the Blue Ensign instead of the Red Ensign.

(This may be the source of the confusion )

The White Ensign (A White field quartered by a Red Cross of St George with the Union Flag in the Top Home Quarter.) Only to be flown by Her Majesties Vessels of War whilst in commision, between Sun Rise and Sun Set.

Very Large White Ensigns (Battle Flags) are traditionally flown when battle or action appears immenant.

There are exceptions to the rule. The Museam Ship HMS Belfast has royal permission to fly the White Ensign. Various Yacht Clubs have the privaledge, (with warrent) to fly a blue ensign, defaced with the badge of the Yacht Club. Trinity House Vessels fly a varient ensign, as to vessels of the Royal Airforce.

I suspect the confusion may be down to White Star Vessels having warrents to fly the Blue Ensign. I also suspect that these vessels were regulally inspected by the Admiralty as a means of keeping the Contingency List of Vessels for requisition up to date.

After the losses of the "Jervis Bay" and "Rawalpindi" in action most Armed Merchant Cruisers reverted to the role of Fast Troopships.

Gareth