I think it's better to vote for a party which has no chance of winning than to spoil your vote. At the very least it communicates what kinds of policies you would like to see and what policies would win your vote in the future.

I constantly think about the 2015 general election and how UKIP got almost 4 million votes (the third highest number of votes amongst all the parties). I feel that this caused a shift within the Conservative party towards populist, Eurosceptic, and anti-environmental ideals because they realised by doing so they could win back those 4 million voters.

I would personally never spoil my ballot for this reason. I don't think it's especially valuable to communicate that you're not happy with anything without communicating what would make you happy.

I'm currently in a circular debate with myself as to whether to vote Labour or Green. The classic eternal debate of "splitting the left vote" which we must deal with since we use an archaeic First-Past-The-Post system which should not exist in any modern democracy. I don't even especially like the Greens but a vote for them may communicate that one of my biggest values is preserving the environment and tackling climate change. Perhaps this could encourage Labour to establish policies to address these things in order to win back Green votes.