As Mark has pointed out, the police rowed back from listing XR as extremists, claiming that this was a mistake. However, it's important to note that the UK is extremely proactive in banning or refusing entry or visas to people considered undesirable.
At various times in the last few decades anti-vivisectionists, euthanasia activists, and numerous far right activists have been excluded from the UK, even in the absence of any criminal conviction.
Border officers have the power to refuse entry if they believe that an individual is "not conducive to the public good". I think it quite possible that if a border officer came to believe that you were travelling to the UK to participate in protests, especially those which involve significant disruption, they would refuse you entry. Your mere association with XR could cause the border officer to believe you were coming to the UK to protest.
Given the heavy handed way in which the XR protests in London were policed, the previous use of anit-terror legislation against climate activists, and the right wing ideology of the current government and particularly the Home Secretary, it's entirely possible that a future XR protest could cause the government to treat XR activists harshly, and to attempt to exclude foreign activists from the UK.
I'm not saying that you need to be particularly concerned about your future travels to the UK, but I would avoid carrying any kind of XR material on your person when you enter the UK, to avoid discussing your activism with the border officer.