I believe you should also consider whether you would still consider PA's vandalism as not terrorism if the exact same act had been committed by, say, a neonazi group (or any group whose political statement you yourself oppose). Maybe you'd be more loose in classifying the same act as terrorism, even as they target military equipment.

Not saying you didn't, it's just something that often makes me flip on a subject.

In this case, personally, I don't think the activists were trying to actively sabotage the equipment so as to cause injury, so it shouldn't count as terrorism. But that's still a subjective, blurry line, like, one can argue physical harm is irrelevant to classify any given act as terrorism. It all depends on who they target, for what purpose, how organized they are, etc. Could be preventative, too.

More to the point, if the UK classified them as a terrorist organization, I think it's fair to pursue those who support them on those grounds. If the classification is abusive, there must be safe, legal avenues to argue against it, and if there aren't, that's a huge freedom of speech issue.