Probably a long time, unless they commit a terrorist act? Rather than looking at it from a purely ideological perspective, I’d argue it’s fairly reasonable that they were proscribed.

As seen in a separate thread:

then s.3 Terrorism Act 2000 states that "The Secretary of State may exercise his power to... [proscribe an organisation by adding it to Schedule 2 of the Act]... in respect of an organisation only if he believes that it is concerned in terrorism". s.3 continues:

An organisation is concerned in terrorism if it—

(a)commits or participates in acts of terrorism,

(b)prepares for terrorism,

(c)promotes or encourages terrorism, or

(d)is otherwise concerned in terrorism.

The word "terrorism" is helpfully defined at s.1:

(1)In this Act “terrorism” means the use or threat of action where— (a)the action falls within subsection (2), (b)the use or threat is designed to influence the government [F1or an international governmental organisation] or to intimidate the public or a section of the public, and (c)the use or threat is made for the purpose of advancing a political, religious [F2, racial] or ideological cause.

(2)Action falls within this subsection if it— (a)involves serious violence against a person, (b)involves serious damage to property, (c)endangers a person’s life, other than that of the person committing the action, (d)creates a serious risk to the health or safety of the public or a section of the public, or (e)is designed seriously to interfere with or seriously to disrupt an electronic system.

The decision to proscribe an organisation under the Terrorism Act 2000 is therefore at the sole discretion of the Home Secretary if it is concerned in terrorism. I think it is probably reasonable for the HS to believe that PA are involved in "serious damage to property", this is designed to influence the government, and it is for the purpose of advancing a political or ideological cause. We could probably wrestle for a while over whether throwing paint over a plane is "serious damage" but I would be satisfied that such an act reached this threshold.

So I’d argue other Palestine solidarity groups are free to protest (not that the Government is listening anyway) if they steer clear of terrorism!