Well then, Section 4a of the Public Order Act requires intent:
(1)A person is guilty of an offence if, with intent to cause a person harassment, alarm or distress, he—
(a)uses threatening, abusive or insulting words or behaviour, or disorderly behaviour, or
(b)displays any writing, sign or other visible representation which is threatening, abusive or insulting,
Whereas Section 5 doesn't:
A person is guilty of an offence if he—
(a)uses threatening words or behaviour, or disorderly behaviour, or
(b)displays any writing, sign or other visible representation which is threatening within the hearing or sight of a person likely to be caused harassment, alarm or distress thereby.
So, Section 4a "I upset you and meant to". Section 5 "I upset somebody and I didn't necessarily mean to."