What is the most ridiculous thing you can do to a Buckingham Palace guard to which they still wouldn't be able to display a reaction?

I was watching an episode of Mr. Bean where he totally screws with the palace guard. I assume they’d stop any threats (they are actual guards after all), but how far can you push one of these guys without retaliation? Like, could I not brush my teeth for days and breath in his face for 10 minutes? Not that I have any desire to mess with him, I just wonder what they can tolerate before they smile/yell/kill.