https://www.theguardian.com/notesandqueries/query/0,5753,-23636,00...
The Metropolitan police's web site (http://www.met.police.uk/history/oldbill.htm) suggests a number of derivations. the most plausible is: The London County Council at one time registered all police, fire and ambulance vehicles with the letters BYL The earliest documented usage traced by the ...