It wasn't until the 19th century that Britain had a police force as we know it today. In medieval times, the maintenance of law and order was in the hands of local nobles and lords who were expected to keep the peace in their own land, and they would often appoint "constables" to police it. For a long time policing remained an unpaid activity or was paid for privately, either by individuals or organizations. There were also people who made a living as "thief takers". They were not paid wages, but were rewarded by a proportion of the value of the stolen possessions they recovered. Later, in London, where the population was rapidly increasing and crime was rising, night-watchmen - the first paid law enforcement body - were created and worked alongside the unpaid, part-time constables.
Britain, then, was slower to create and develop a police force than the rest of Europe: France had one long before - indeed, the word police is taken from the French. This fact was not unimportant, as the very idea of a police force was seen as foreign - that is, French - and particularly undesirable, and was generally regarded as a form of oppression.
It was not until Robert Peel set up his "new police" as a separate force in 1829 that policemen began to replace the old part-time constables. Sir Robert "Bobby" Peel's own name provided two common nicknames for the new force: "Peelers" or "Bobbies". These names seem mild, if not affectionate, and are possibly an interesting gauge of how the police were viewed by people at the time, in contrast with the kind of names they get called these days.