For any non-Brits, the SWP is the main left of Labour party in Britain. As a student in the 80s I remember them as an earnest, way too serious bunch of ideological zombies ranting into microphones at the Student's Union about the latest crimes against humanity perpetrated by 'Thatcher'. They're the kind of people that come up to you in the street with leaflets about some atrocity in Africa and shout "Would you murder children" at you. I'm not entirely sure they'd recognise what an actual 'Worker' was if they met one. I always found them quite comical.
However they are a serious party, they're a highly motivated and active movement with significant influence in the Labour Party grass roots. They're sort of our equivalent of Antifa in that they are a significant organising presence in lefty street protests, but much more politically active and spend a lot of time in conferences and action committees wondering what Capitalism ever did for us.
Importantly, they are in no way shape or form a terrorist or criminal organisation. I would not be at all surprised if some ultra-violent or would-be Red Army Faction types joined or associated with them, but that's not at all who the SWP are as an organisation. They're the well meaning, impressionable white middle class girl next door with a class guilt complex and a side shave that's been going to BLM protests.
If they are getting banned by Facebook something has gone horribly wrong. Quite possibly some of them got a bit excited and posted things they shouldn't, so I'm not saying this is for sure Facebook's fault, but they're not about to murder any bobbies in Parliament Square.