I'd say that not voting (or better still voting but spoiling your ballot paper) can make a more powerful statement than picking the least-bad option or wasting a vote in a safe seat where it's not going to make any difference to anything.

Voting for any of the candidates is stating that we're content with the voting system that we have.  At present, most people's votes make absolutely no different to who their MP or prime minister is, so they may as well not bother.  Democracy only exists in the minority of seats where there's a narrow margin between the parties.