Would adding or removing incorrect punctuation or spelling on signs and notices be counted as a crime?
Potentially yes. Defacing a sign or a notice belonging to someone else could well be considered criminal damage, even if it were to correct a mistake. The so-called "Grammar Vigilante" who corrects signs around Bristol was asked if he considered his activities a crime, and said: "It's more of a crime to have an apostrophe wrong in the first place." This is not true.
Does the attack when a man drove a bus into worshippers at Finsbury Park mosque count as terrorism?