http://www.theroyalforums.com/forums/newreply.php?do=newreply&p=1223268
As I thought - it was the use of the word 'courtesy' that was confusing the issue - not our understanding of the actual issue.
However, the Sovereign can also strip a peer of that title as well. As Peerages are also granted by LPs they can just as easily be stripped e.g. The Titles Deprivations Act.
A peer can only be deprived of his titles by an Act of Parliament, which holds true today just as in the past.
The Titles Deprivation Act can only be employed to strip the titles and/or royal dignity of a person who served or is serving in an enemy military force, has rendered assistance to, or voluntarily resides in an enemy nation.
This was the case in World War I, when many of the German royal families were more closely related to the BRF, and for which the Act was approved by Parliament.
But even though the four men targeted by the Act were deprived of their British titles and dignities, their successors would be allowed to (and can still) petition the British government for a restoration of those titles.
The peers who were stripped of their titles in 1917 were:
HRH The Duke of Saxe-Coburg und Gotha (Duke of Albany, Earl of Clarence, Baron Arklow
and Prince of the United Kingdom)
HRH The Crown Prince of Hannover (Duke of Cumberland and Teviotdale, Earl of Armagh
and Prince of the United Kingdom)
HRH The Duke of Brunswick (Prince of the United Kingdom)
The Viscount Taaffe (Viscount Taaffe and Baron Ballymote)
To date, none of the successors of these men have petitioned to have these titles restored.. and at this point, I believe it is unlikely to happen. But I also doubt the BRF will grant the affected titles to anyone else as long as there is a potential legitimate heir still alive.
The Taaffe title became extinct in 1967. The rest continue to have living heirs.