Retired and Former Officers
Field marshals remain on the active list for life and so continue to use this rank. Other regular officers of the rank of major and above may use, and be addressed by, their rank after being placed on the retired list.
It is not considered appropriate for more junior retired officers to continue to use their rank; this advice also applies to temporary officers of all ranks (e.g. those with war service only).
Usage has it, however, that retired cavalry officers of the rank of captain, who continue to work in civilian life with horses (both in racing and in equestrian sports) retain their rank (e.g. Captain Mark Phillips).
The word 'retired' (or retd) should not be added after an officer's name in ordinary correspondence or in lists, but only when it is specifically necessary to indicate that an officer is on the retired list, for example one employed in a civilian capacity in a Ministry of Defence establishment, when it facilitates postal arrangements.