As commented above, yes, it is possibly illegal under UK law. For example:
(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 thereby causing that or another person harassment, alarm or distress.
(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 thereby causing that or another person harassment, alarm or distress.
http://en.wikipedia.org/wiki/Intentional_harassment,_alarm_o...