Although the UK doesn't have a constitution, Freedom of Speech is guaranteed in Article 10 of the Human Rights Act 1998 and protected by the European Convention

Although the UK doesn't have a constitution, Freedom of Speech is guaranteed in Article 10 of the Human Rights Act 1998 and protected by the European Convention on Human Rights. That being said, the number of clauses in which freedom of speech is 'restricted' means that the UK doesn't strictly have freedom of speech. You are even free to speak as you want or not. The UK falls into the latter category.