I tend to agree, I remember Blair trying to claim that low turnout was because people were happy. Much harder to spin 'your all scum' scrawled on a ballot paper in the same way.
On the mandate issue, spoilt ballots reduce the majority of the winning party and therefore reduces heir legitimacy as hey have secured less of the votes cast. What we need hough is a campaign to get people to write the same message on their ballot paper. May I suggest the following: 'down with parliament, down withe bourgeois democracy, forward to international working class revolution.'
Usually though I am too lazy to go to the polling station to bother.