I don’t follow UK politics with anywhere near the same detail as I do the US situation, but I’m yet to see anything from Corbyn that I’d classify as “antisemitic” rather than “critical of Israeli policy towards Palestine”.
And to quote a random Twitter leftie:
Complicated question.
There was a purge of dodgy stalker blokes from the online left a few months ago (about a month before #MeToo blew up). There are semi-regular instances of online Strasserists being told to fuck off. And, of course, there’s the constant squabbling between the Anarchists and the Marxist-Leninists.