The following are extracts from the Metropolitan Police Orders dated Thu February 7th, 1884 Tue, February 10th, 1885 & Wed, May 25th 1887.

Appointments - Whistles. - The whistles issued for the use of Police serving in the exterior Divisions, are to be fastened by chain hook to the button hole of the great coat or tunic, the whistle being placed under the garment in such a manner as will admit of it being readily used when necessary. The instructions for the use of the Rattle as a means of alarm apply to the use of the whistle.

Appointments - Whistles - Rattles -1. Referring to General Orders, par 1, a whistle will in future be issued to each police officer as part of his equipment, in lieu of a rattle.
2. Requisitions are to be submitted, 1st April, on Form 90, for a sufficient number of whistles to complete the supply of one to each officer.
3. Each sergeant and constable will, when parading for night duty, be supplied with a rattle, which he is to deposit at the Police Station when going off duty.
4. The Superintendents are to carefully examine the rattles now in their Divisions, and select a sufficient number of the best of them to meet all requirements and afford a surplus for emergencies.
5. Returns are to be submitted, 16th, shewing the number of rattles recommended to be retained in each Division, and the number to be sent into the receiver's store.

Therefore it can accurately be determined that February 1884 was the month the order came for the introduction of the general service whistle into the Metropolitan Police Force, followed by instructions of how the whistle was to be worn. In modern times the whistle was to be placed into the top left tunic pocket.

Whistles. -1. Whistles are to be carried by Police at all times when on duty, and in the following manner.
2. The hook is to be fastened in the second button-hole from the top of the tunic or great coat and whistle placed between the buttons, the chain to hang nearly straight on the outside of the garment.