So in that list, that leaves the Greens or abstination.
I was going to say that the Green party was just a single-track party, but on checking up it seems they do actually have policies beyond [misguided] environmentalism. Not yet determined whether I agree with them, but I guess I'll take a closer look when the election actually turns up and consider whether to spend the futile effort on voting for the Greens rather than abstaining.
I think it's more that they didn't do anything at all.
They, unnecessarily, sided with the (from the point of view of a leftie voter) most evil party with the intention of making them slightly less evil. Rather than siding with a good party and getting some good done.
And then they singularly failed to make them less evil. Because the Tories are very fucking good at politicking. They know how to play the media and make sure that the LibDems lose even the small amount of influence they should've had.