Is a video clip of JK Rowling exploding, posted on twitter a threat of violence?

I think it's nasty and borderline abusive, but unless it's calling for it to happen or threatening they will actually make her explode, then I'd not describe it as an actual threat.
Lucy Connelly's tweet was a very different thing. She was literally calling for hotels to be set on fire in the middle of ongoing riots.
I don't think this is the same thing at all.
I think Prior is a creep and posting a video like that shows his true colours, but I don't think it is actual incitement to violence.

Just been on X to have a look. It’s an animation of the cigar-on-a-yacht pic of JKR dissolving to background singing of fuck you. Agree it would probably be satire and I don’t read is as a threat (that said don’t know anything about Mr Prior so happy to take this back if he has form).

One of his tweets, while I was searching for the offending one, was about a publication called Wizarding World which is apparently saving the HP universe from that dreadful JKR. 🤦‍♀️ I can see where she got the inspiration for some of her characters and activities in Strike now.

I doubt it - probably counts as satire...

Let's remember that many of the responses to tweets and social media are way over the top - there is a lot you can do online - we technically still have free speech as a concept, though it becomes more doubtful by the day...