I hope you’re right because this article says they used a spray can.
Which brings me back to the last point in my comment.
I also hope I'm right. The two times I looked into it (right after the attack and before writing my comment) both came up with that result. Also it seems that English Heritage came out today saying there was "No visible damage".
As I said, I'm not writing to defend the action, just pointing out that the OP article is, willfully or not, omitting certain aspects that could make JSO look a little bit better.