you can show your disagreement by simply casting your ballot without ticking the box

The "None of the above" option has to be clearly distinguishable from "didn't operate the mechanism properly". There is a separate "spoiled ballot paper" count in UK elections, but this covers everything from obscene graffiti to a blind person missing the form. In elections with "write in" options, I suppose one can already specify "none of the above". At university, we had write in votes for student council elections. It was a tradition to vote for King Farouk (don't ask me why), who was quite often duly elected and then disqualified, forcing a new election.

Rod