Palestine action group are the group that broke into a UK military base and caused millions of pounds worth of damage.
"The proscription of those three organisations will reaffirm the UK’s zero-tolerance approach to terrorism, regardless of its form or underlying ideology.
It may be helpful to set out some background to the proscription power. To proscribe an organisation, the Home Secretary must reasonably believe that it is concerned in terrorism. That means that the organisation commits or participates in terrorism, it prepares for terrorism, it promotes or encourages terrorism, or it is otherwise concerned in terrorism. Some 80 terrorist organisations are currently proscribed under the Terrorism Act 2000.
Proscription is, rightly, ideologically neutral: it judges an organisation on its actions and the actions it is willing to deploy in pursuit of its cause. The UK’s definition of terrorism was established in law a quarter of a century ago, and it has stood the test of time and extensive scrutiny since."
Note - there were 3 organisations proscribed - nobody appears to care about the others though.
"Focus on National Security:
The government's decision to proscribe the group appears to be largely driven by concerns about the serious damage to property, particularly that related to national security, rather than solely focusing on violence against individuals."
Property involved in defence of the nation is actually fairly important. I can see why a country might want to stop that. Not sure why others struggle to see it is important for a country to have it's defence ready and able to go if needed.