MPs have voted to proscribe Palestine Action as a terrorist group

Normally terrorism involves attacking/harming/killing civilians though, right? And Palestine Action didn't do that. They just sprayed some paint on a plane.

You could say "trespassing onto a military base and damaging a plane should be punished", sure. But it's not terrorism in the same sense as most things that are labelled as terrorism (e.g. 9/11, 7/7, Bataclan massacre, etc).