Uk Flag Etiquette

[Nausikaa]

Here is one for the Britishers and the vexillologists amongst us.

A vessel visiting the UK should normaly fly the red ensign at the starboard yardarm of the formast. I have, however, during the Vietnam war, seen a US warship flying the white ensign at the yardarm, which is unusual as warships do not normaly fly courtesy ensigns. Nevertheless, it brings me to my question, a yacht beöonging to a royal yacht club of another country when visiting the UK should fly what as a courtesy ensign? Obviously, a red ensign will not be wrong but could she, as a vessel of a royal yacht club (which in British terms would normaly have a defaced blue ensign) fly a blue ensign as a courtesy ensign?

Also, which ensign should a foreign state owned vessel (such as a coast guard cutter, customs vesel etc.) fly when visiting the UK?

Any ideas?

Aye // Stephen