Not sure if OP or anyone else who thought along similar lines will see this follow up, but I was mulling the original comment over some more and I know why I reacted so strongly to it.
When I think of the word terrorism, and I'm sure many others think like me, I think of 9/11. Or the Las Vegas shooting. Or one of the many horrible bombings in various parts of Middle East, etc.
Those are acts that have no pre-warning, and are designed to inflict as much pain and suffering as possible, and to cause as much fear and, dare I say it, terror as possible.
What XR are intending on doing is abusing an obvious outcome of notifying civil aviation authorities that a dangerous object will be active in airspace where civilian commercial airflight is taking place. The outcome being flights are grounded and redirected, causing a lot of inconvenience to people and costing the economy potentially millions of dollars.
I think XR believes it's a 100% obvious thing that by notifying the appropriate authorities well ahead of time, all planes will be safely rerouted or grounded. If there was even the slightest chance that their actions could cause a plane crash, then I firmly believe XR would not even consider such an action.
So yes, I understand we have actual legislation that could very reasonably be used to charge the people responsible for the plan at Heathrow airport. The problem is I think that the legislation in question was designed to target something very different. If activists from XR are convicted of charges stemming from the "Terrorism Act", it becomes perfectly reasonable to label those activists as terrorists.
But then what we've actually done here is placed activists who never had the slightest intention of causing grievous harm to other people (loss of life, massive economic damage, etc) into the same category we reserve for extremists who have perpetrated genuine atrocities. That just seems completely wrong to me.
If we want to bring about legislation that criminalizes certain forms of activism that go "too far", then fine. I think even that could be a perilous path, but I can make my peace with it being necessary. I would just hope we stop and ensure everyone is on the same page that we're categorizing these people as something like "activists who went too far" in order to distinguish them from "extremists who carried out attacks that we as a society consider utterly unthinkable and terrifying".