There is a basis in UK law for freedom of speech (most recently, Article 10 of the Human Rights act). It's true that protections for free speech are not as extensive in the UK as they are in the US, but the US is the outlier in that case. Very few countries have free speech protections as strong as the First Amendment.
The UK laws elaborate on what is and isn't free speech, while the US law basically just says "there shall be free speech (as far as Congress is concerned. Other parties can do whatever they like to stifle speech)"
UK law is extremely loosely defined. Judges are ultimately responsible for its interpretation, which they do relatively literally - so as long as the police and CPS bring a case there's a good chance you've fallen foul of the law, subjectively - which is how they are written. E.g malicious communications act.
The US constitution is even more loosely defined if you exclude the outcome of hundreds of years of judicial interpretations of it. Hence the endless disagreements over what is or isn’t constitutional.
Which must be tosh, you can literally go to prison for years here for stating facts. What if some day somebody takes offence to 2+2=4? The guy who says 2+2=4 goes to jail for years whilst rapists and murders get away with 6 months or suspended sentences. But don't mention that, or else!
