Peter - try informing yourself. HM Government has a website which outlines what it considers 'Prevent' is all about: https://www.gov.uk/government/publications/prevent-duty-guidance

Nothing there specifically about 'Islamic extremists' or 'the far right' - it's all about preventing people getting into 'terrorism'. So if self-identifying 'communist' numbskulls start prattling on about armed struggle and suchlike nonsense on social media, they run the risk of coming under the 'Prevent' radar. Of course, most sensible people would realise that it's all 'rrrevolutionary' talk by muppets who probably wouldn't know one end of a gun from the other. But there's always the danger that someone less dismissive than me might take these fantasists seriously...