A cop is a person. They work for the police. Cop is an informal word for police officer.

They mean the same thing. Police is considered a little more respectful but cop is not a bad word.

I believe that Cop is in the UK and Police in the US, same meaning, different country.

Cop is just colloquial for Police officer. It used to be a bit derogatory, but that was back in the 50‘s, maybe 60‘s. Now it’s pretty mainstream and common.

Cop would be a better replacement word for the phrase Police Officer, rather than the Police Force. While Cop (or Copper) is a slang word, and be considered as such in your useage of it, as the others have pointed out here, it is not a disrespectful word. You wouldn't get into trouble for using it.