I was curious about the "blank sign" story because it's slightly different from what I remembered reading. As far as I can tell, this is the incident you're referring to:

    On 12 September, Charles addressed parliament as king for the first time. The Metropolitan police called in reinforcements in case of protests. Powlesland, who works nearby, walked from Parliament Square to Downing Street and back with his blank piece of paper. “Then a guy from Norfolk police came up and spoke to me, and that was the video that went viral.” Powlesland recorded the encounter on his phone. “He asked for my details, I asked why and he said, ‘I want to check you’re OK on the Police National Computer.’ I said, ‘I’ve not done anything wrong, so I’m not giving you them.’ I wanted to test it without getting arrested. So I asked, ‘If I wrote “Not my king” on the paper, would I get arrested?’ and he said, ‘Probably, because it would be a breach of the Public Order Act; it would be offensive.’” Was he right? Powlesland laughs. “No! Just having something someone else finds offensive is not a criminal offence because then pretty much anything could be.”