There's literally nothing tyrannical about this situation.

On one hand, you have a democratic country, the UK, that has voted in a set of well defined laws that clearly outline what terrorism means, what can be considered as terrorism, and what the consequences are for terrorist acts. These legal parameters have been established law for decades.

On the other hand, you have an activist group, Palestinian Action, that knowing and intentionally chose to violate these laws by illegally sneaking into an RAF base and intentionally damaging military equipment for political purposes.

Considering how this is a malicious act of sabotage, a breach of national security, and an attempt to undermine the British state, this organization fits the criteria to be designated as a terrorist group. Because of this British politicians, quickly employed the established terrorism laws to give them the designation. Not only because what they did is considered terrorism in the country, but also because they don't want to set a precedent that this type of action gets a pass. They wanted to make it clear that such actions are an unacceptable red line, and those who cross will be swiftly punished.

So in essence. There were laws established democratically, a group intentionally violated them, and they're now facing the consequences of doing so. The UK is not censoring their activism cause, they're going after other groups that didn't do anything wrong, and this group is not being punished over any free speech grounds. Calling this tyranny is ignorance.