Don't spoil your ballot paper, vote for the next party you favour, whether Lib Dems or Greens or independent. Others will think like you and that might help in its own small way. Imagine being the greens and seeing your vote share slowly increase. It inspires. The SNP started on hardly any votes and had to give it time to take off. That doesn't always happen and it can be incredibly hard to break into what is really a two party system, but most of us will be alive for the next 50-60 years and this is just one election of many to come.
If you're purposely going to just draw a dick on the ballot paper you may as well just stay home. I mean, I know you're not drawing a dick, at least I hope not, but it's the same effect if that's your goal to chuck the vote.
While it is likely you'll be in an area that should vote Labour to oppose the Tories, some of England is best to actually vote Lib Dems
http://www.tactical2017.com