"Constable" the traditional appointment for the role of safeguarding the community and property.
There have been "Constables" since the Medieval times who were appointed locally by magistrates
As the Police Forces became more formally organised "Constable" became a rank within the service
A Police Officer is really a generic term for any Police Service member, the rest are rank, such as Police Constable, Detective Constable etc
Bizarrely the "Constable" as part of the rank is used in the lowest rank,and then returns again in the higher ranks with Chief Constables etc