I didn’t mean to say say sabotage could not be terrorism in general, I’m agreeing that this is absolutely sabotage, but in this case it is not terrorism. The govt do not have a valid reason to proscribe them as a terrorist organisation. There are other available methods of opposition.
Terrorism is generally linked with a risk to personal safety at the very least, not mere property damage. These people are criminals, not terrorists.

The nuance here is that they’ve been proscribed as a terrorist group. They are a direct action group but they don’t hurt people, just damage property. That’s a crime, but it’s not terrorism. They can and should be tried for criminal damage, B&E, damage to government property etc, but this is a freedom of protest issue, and now freedom of speech, too, since I can no longer say things like “I think Palestine Action went too far, but they’re fighting for a just cause”, for example, because I’d be expressing support for a terrorist group and therefore committing a crime.
Compare to e.g. the suffragette movement who bombed buildings and all kinds to get the vote for women: they’re now lauded by the same person who proscribed this group. Or more recent examples like just stop oil or the protesters that threw the Edward Colson statue into the River Avon. They were tried for criminal damages. Then recently, some protesters have been starting to get tried under the far, far harsher anti terrorism laws, usually when it involves protesting Israel’s war crimes. These are the laws that allow things like extended detention in solitary without charge or phone calls, massively longer sentences and all manner of nasty punishments. It’s clearly a power grab to reduce the will to protest.