The law on threatening behavior

Whether you scream at somebody or tell them calmly is irrelevant,
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.