Freedom of Speech doesn't exist in the United Kingdom and what rights you do have are greatly diminished online, thanks to the Communications Act 2003 (and, before that, the Telecommunications Act 1984). Although guidance has been issued that seeks to limit its reach, you're still at risk of prosecution for saying anything "grossly offensive or of an indecent, obscene or menacing character" online.
Essentially, this criminalises just about any opinion that a court sees fit to prosecute since, odds are, someone will find it offensive. Since it's all subjective, this leads to the bizarre scenario where both an opinion and its opposite counter-opinion could be deemed criminal if expressed online.