Whether you scream at somebody or tell them calmly is irrelevant, if that is what you are getting at.
It is the Criminal Justice and Public Order Act 1994 that covers it:
This defines a criminal offence of intentional harassment, which covers all forms of harassment, including sexual. A person is guilty of an offence if, with intent to cause a person harassment, alarm or distress, s/he:-
uses threatening, abusive or insulting words or behaviour, or disorderly behaviour; or
displays any writing, sign or other visible representation which is threatening, abusive or insulting, thereby causing that or another person harassment, alarm or distress.
What exactly has happened?