Here's a little something that stipulates that malicious and offensive postings can be deemed as illegal:http://www.legislation.gov.uk/ukpga/2003/21/section/127
127Improper use of public electronic communications network
(1)A person is guilty of an offence if he—
(a)sends by means of a public electronic communications network a message or other matter that is grossly offensive or of an indecent, obscene or menacing character; or
(b)causes any such message or matter to be so sent.
(2)A person is guilty of an offence if, for the purpose of causing annoyance, inconvenience or needless anxiety to another, he—
(a)sends by means of a public electronic communications network, a message that he knows to be false,
(b)causes such a message to be sent; or
(c)persistently makes use of a public electronic communications network.
(3)A person guilty of an offence under this section shall be liable, on summary conviction, to imprisonment for a term not exceeding six months or to a fine not exceeding level 5 on the standard scale, or to both.
(4)Subsections (1) and (2) do not apply to anything done in the course of providing a programme service (within the meaning of the Broadcasting Act 1990 (c. 42)).