Having perused a copy of Debrett’s I found the established practice is:
Anyone attaining the rank of Admiral of the Fleet, Field Marshal or Marshal of the Royal Air Force ‘continue to use this rank’ as they effectively remain on the active list permanently.
Other regular officers who attained the substantive rank of captain and above may use, and be addressed by, their rank on retirement from the Army.
It also states:
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 has retired, for example one employed in a civilian capacity in a Ministry of Defence establishment to avoid confusion with serving officers.
Interestingly it has the same wording for retired Royal Navy & RAF officers but for Lt Cdrs & Sqn Ldrs. Why an ex-army JO gets to retain their rank title when you need to be a SO from the other services it doesn’t explain.