I've taken my training to be a poll clerk in the upcoming election, and I have to correct statements I made earlier about declining your ballot.
It is possible for you to decline your ballot, and the procedure is that you show up at the polling station, announce yourself, accept your ballot, and then hand it back to the Deputy Returning Officer saying "Thank you, but I am declining my vote."
I as a poll clerk will cross your name off the voters' list, noting that you have voted. The Deputy Returning Officer will, without looking at the ballot, deposit it in the ballot box.
This is different from spoiled ballots -- which are ballots we discover with printing errors, or ballots that voters have accidentally marked up (if you make a mistake on the ballot, tell the Deputy Returning Officer; he'll take your old ballot as a spoiled ballot and hand you a fresh one. You get only one chance at a redo, however). These spoiled ballots are placed in a special envelope and are counted outside of the turnout. Declined ballots placed in the ballot box are counted inside of the turnout, but are counted no differently than if you wrote, say, "they're all idiots" on the ballot and turned it in without announcing your intension to decline your vote.
So, declining your vote may feel good as a worthwhile statement of protest, and you will be counted as having come out to vote, but you are counted no differently than somebody who spoils his or her ballot secretly and puts it in the ballot box.