Where should I look for information about a former chief constable or senior officer of a force?

Start with a recent publication, which attempts to provide dates for the careers of chief constables of British police forces: Martin Stallion and David S Wall, The British police: forces and chief officers 1829-2012 (Police History Society, 2011)

The minute books of the relevant police committee should provide some information about the appointment of a chief constable and, in some cases, other senior officers, such as dates of appointment and leaving, and some details about their background, such as their previous constabulary. If further details are required, such as major events during the period in office, more lengthy research might be required in minute books, scrap books, memoranda books and letter books, if any or all of these survive.