Can anyone explain in a basic way, what freedom of speech, allows, and does not allow a person to do?

There is a person who has negatively effected people's lives, and even companies reputations within the UK. She has made a false allegation about me to the police already, so I have to be careful, but I think other people need to know about her, so they do not experience the same problems as what I have. What are my rights in this situation?