Use of an Officer's Rank Without His Surname
In social correspondence certain officers may be addressed at the beginning of a letter by their rank alone, e.g. 'Dear Admiral' in place of 'Dear Admiral Smith', but some officers dislike this practice, especially when used by those outside the Services.
In any event this practice is limited to the following ranks:
Royal Navy: commodore and above (rear-admiral, vice-admiral and Admiral of the Fleet being shortened to 'Admiral').
Army: lieutenant-colonel and above (lieutenant-colonel being shortened to 'Colonel', and major-general and lieutenant-general being shortened to 'General').
Royal Air Force: air vice-marshal and air chief marshal (shortened to 'Air Marshal'), and Marshal of the Royal Air Force (shortened to 'Marshal').