See Section 127 for details.
There have been several high profile cases in recent years where people have been prosecuted under Section 127 of the Communications Act 2003 for comments made on social networking sites. The issue is the difference between simply being "offensive", which is legal, and "grossly offensive", which is not. The distinction is not easily made, as admitted by Director for Public Prosecutions, Keir Starmer, who has called for an open consultation into amending Section 127.
Hate Speech Regulation
Libel Law and Defamation
Question: Will Data Retention cause excessive enforcement of speech laws?