It feels to me like there's a distinction between "on one occasion, one person in group X did Y" and "group X does Y", and it's the second of those that (for some choices of Y, including "attacking police with sledgehammers") could justify calling group X a terrorist group.
Obviously "on one occasion, a person in group X did Y" is evidence for "group X does Y". If Samuel Corner attacked a police sergeant with a sledgehammer during one Palestine Action, er, action, then that's the sort of thing we expect to see more often if PA is generally in favour of attacking police with sledgehammers. (Either as a matter of explicit open policy, or as a nudge-nudge-wink-wink thing where everyone in PA knows that if they start smashing up police as well as property then their PA comrades will think better of them rather than worse.)
But it falls way short of proof. Maybe Samuel Corner sledgehammered a cop because Palestine Action is a terrorist organization after all; but maybe Samuel Corner sledgehammered a cop because Samuel Corner is a thug or an idiot or was drunk or whatever. Or maybe Samuel Corner sledgehammered a cop because the cops were already being violent with the Palestine Action folks and he was doing his (ill-advised) best to protect the others from the police. (This, as I understand it, is his account of things.)
(An Oxford University graduate attacked a police officer with a sledgehammer. I take it you would not say that that makes the University of Oxford a terrorist organization, and you wouldn't say that even if he'd done it while attending, say, a university social function rather than while smashing up alleged military hardware. It matters how typical the action is of the organization, what the group's leadership thinks of the action, etc.)
I took a look at the video. It's not easy to tell what's going on, but it looks to me as follows. One of the PA people is on the ground, being forcibly restrained and tasered by a police officer, complaining loudly about what the police officer is doing. (It isn't obvious to me whether or not her complaints are justified[1].) There is another police officer, whom I take to be Kate Evans, nearby, kneeling on the ground and helping to restrain this PA person. Samuel Corner approaches with his sledgehammer and attacks that second police officer. I can't tell from the video exactly what he's trying to do (e.g., whether he's being as violent as possible and hoping to kill or maim, or whether he's trying to get the police officer off the other person with minimal force but all he's got is a sledgehammer).
[1] I get the impression that she feels she has the right not to suffer any pain while being forcibly restrained by police, which seems like a rather naive view of things. But I also get the impression that the police were being pretty free with their tasering. But it's hard to tell exactly what's going on, and I imagine it was even harder in real time, and I am inclined to cut both her and the police some slack on those grounds.
It's highly misleading, even though not technically false, to say that Corner attacked Kate Evans "while she was on the ground"; she certainly was on the ground in the sense that she was supported by the floor, and even in the sense that she wasn't standing up -- I think she was crouching -- but it's not like she was lying on the ground injured or inactive; she was fighting one of the other PA people, and she was "on the ground" because that PA person was (in a stronger sense) "on the ground" too.
For the avoidance of doubt, I do not approve of attacking police officers with sledgehammers just because they are restraining someone you would prefer them not to be restraining, even if you think they're doing it more violently than necessary. And I have a lot of sympathy with police officers not being super-gentle when the people they're dealing with are armed with sledgehammers.
But the story here looks to me more like "there were a bunch of PA people, who had sledgehammers because they were planning to smash up military hardware; the cops arrived and wrestled and tasered them, and one of the PA people lost his temper and went for one of the cops to try to defend his friend whom he thought was being mistreated, and unfortunately he was wielding a sledgehammer at the time" than like "PA is in the business of attacking cops with sledgehammers".
None of that makes Kate Evans any less injured. But I think those two possibilities say very different things about Palestine Action. Carrying sledgehammers because you want to smash equipment is different from carrying sledgehammers because you want to smash people. Attacking police because they are a symbol of the state is different from attacking police because they are attacking your friend. One person doing something bad in the heat of the moment because he thinks his friend is being mistreated is different from an organization setting out to do that bad thing.
There are plenty of documented cases of police being violent (sometimes with deadly effect) with members of the public. Sometimes they have good justification for it, sometimes not so much. Most of us don't on those grounds call the police a terrorist organization. Those who do say things along those lines do so because they think that actually the police are systematically violent and brutal.
I think the same applies to organizations like Palestine Action. So far as I can tell, they aren't systematically violent and brutal. Mostly they smash up hardware that they think would otherwise be used to oppress Palestinians. (I am making no judgement as to whether they're right about that, which is relevant to whether they're a Good Thing or a Bad Thing but not to whether they're terrorists.) Sometimes that leads to skirmishes with the police. On one occasion so far, one of them badly injured a police officer. It's very bad that that happened, but this all seems well short of what it would take to justify calling the organization a terrorist one.