That's not why they have been designated a terrorist group. Many of their members have been shown, after Police investigation, to be supportive of the terror group Hamas, which IS an illegal offence in the UK. THAT is whay Palestine Action has been declared a terror group.
Granny and Blue Haired girl might not be supportive of Hamas personally, but if they ARE supportive of a group which has been proven to be complicit to be supportive of a banned terror group i.e Palestine Action, they are seen under UK law as being equally guilty. They know this, so stop, or face prosecution by the law.
Their choice.