In the U.K. when police arrest someone, they seem to first announce the current time in military/24 hour format, and then they declare the reason or grounds for the arrest. Declaring the grounds for the arrest makes perfect sense. But what is the reason for the obligatory talking clock routine?
There are clear rules about the length of time a suspect can be held.
The UK government says in Being arrested: your rights
How long you can be held in custody
The police can hold you for up to 24 hours before they have to charge you with a crime or release you.
They can apply to hold you for up to 36 or 96 hours if you’re suspected of a serious crime, such as murder.
You can be held without charge for up to 14 days If you’re arrested under the Terrorism Act.
So it is vital information for interested parties, such as family and legal representatives.