Acronym question

Strickland is a Deputy Assistant Commissioner while DCI stands for Detective Chief Inspector. In the case of DAC it seems to be a title used only by the Metropolitan Police - the same rank in the City of London Police is Assistant Commissioner (Assistant Commissioner for the Metropolitan Police would be one rank higher than DAC.)

Metropolitan ranks are:

Police Constable
Sergeant
Inspector
Chief Inspector (Sasha Miller)
Superintendent (Sandra Pullman)
Chief Superintendent
(Abbreviated a D is added to the rank for detectives.)

Then there are the chief officers:

Commander
Deputy Assistant Commissioner (Strickland)
Assistant Commissioner
Deputy Commissioner
Commissioner