Until the last election, I had never voted for any of the 3 main parties in the last 30 years. I voted LibDem last time. Brown wasn't a good Prime Minister. And I was unimpressed with Cameron in oppostion This time, I cannot vote for the Conservatives or LibDems because they have not been very good over the 5 years. Labour are a mess and I am pro Europe, so UKIP won't get my vote.
I believe everyone should vote, so even though the Greens aren't perfect, they are a lesser evil.
That is asuming we get a candidate this time, at the moment none of the parties other than the Labour incumbant have announcy their candidacy.