From what I can see, the Cenotaph wasn't desecrated.
He didn't stand on any of the wreaths that had been laid. In fact, I can see that he was trying to not stand on any of them.
I don't like XR, I think they are an utterly repugnant group and, given their radical energy agenda, it is ironic that they would highlight that climate change means war, when their own vision will also result in conflict over rare earths and other resources to manufacture the kit.
With that said, I think there's quite an overreaction here. I don't see the fallen being disrespected, nobody has attempted to burn the flags, draw graffiti on the Cenotaph or climb on it which is a stark contrast to the events in the summer. The dude just laid a wreath, without standing on any of the others.
Those we are remembering today made the ultimate sacrifice for King/Queen and Country, in the defence of our freedom. One of those freedoms is the freedom of expression, and, while the group is misguided, it is absolutely reasonable to highlight that if we don't tackle climate change now, we will only see an increase in conflict. They have a right to a peaceful protect, just as much as you have the right to call them a bunch of d*cks for doing it on this day. That's how freedom works and I wouldn't want to live anywhere with lower standards.