One of the weirdest motivations for voting for extreme-Right parties is when people say they feel betrayed by the Labour Party. OK, let's just assume you're right and they haven't taken good care of the working man's interests. Your response to this is to... vote for a party that will reliably make the Conservative Party's leader Prime Minister. You aren't terribly smart, are you? Are you really sure you should be voting at all?
@mrundkvist
When it comes to voting there's not a lot of options for how to punish the party that betrayed you: by construction a proportional system only allows you to either vote for the "traitors" or their opponents, there's no "half voting" or other mechanisms for expressing conditional support.
And in any case, in your above mode of reasoning there would be no recourse: even conditional support will easily be described as "stupid self sabotage" as it leaves the labour side weakened.
In truth, the single vote of support is a very blunt instrument for expressing approval.
Thus, if you restrict the possible actions to just the vote, the counterintuitive conclusion that you have to vote against your interests to bring about desired change (over the long term) is actually unavoidable.
But of course there are other ways to affect party behaviour beyond the national vote, though those take more effort and are frankly quite denaturalised in comparison.
@tobychev I say join a party if you're not happy with the menu. Otherwise a person is just a political vandal.