Writing to the police is not an everyday occurrence so when you need to send a missive to a chief constable, you want to use the correct terms of address. In Britain, the position of chief constable is important as the title represents the head of each British police force, except the Metropolitan Police and City of London Police, which have police commissioners. When corresponding with a chief constable, there is a formal way of address and a less formal approach. Both are fine, according to Debretts, which prides itself as an authority on all matters of etiquette and taste.
Determine if the police constable is knighted as many of them are. If so, address the envelope accordingly, such as "Sir Mike Smith." If not, "Mike Smith Esq," followed by any decorations, such as an OBE or other awards, will suffice. Alternatively, make the address simple and less formal: "Chief Constable, Oxford Constabulary" is a suitable address.
Write "Dear Chief Constable" at the beginning of the letter. Another option is to write "Dear" followed by the chief constable's name, as in "Dear Mr Smith."
End the letter with "Yours sincerely" and your name.
Determine if you really need to send a letter to an individual chief constable. Most police forces have a main point of contact and an address for complaints.