An election is coming, so might as well start early.
People often confound voting with betting. They want to back the winner. But no, this is not what is happening. You are doing it to express your tiny part of the popular will. Under First Past the Post (the british electoral system) it is mainly just an expression: only in marginal seats does the vote make a difference to the outcome. Nothing changes in safe seats. For safe seats, read rotten boroughs.
So the question is - are you in a safe seat or a marginal? If in a marginal, you may wish to vote against Labour because of Blair, and against the Tories because they are Tories. If perchance you have a LibDem or Scots or Welsh nationalist candidate (in Scotland or Wales respectively, of course) who is in a position to win, you may be able to help swing things.
But - BUT - if you are in a safe seat, nothing you or anyone does is going to change things. So then you can vote Green because of the total rightness of their thinking vis a vis human survival.
No matter who you vote for, the Gpovernment always gets in. So vote Green - they never get in.