People are often faced with a choice where none of the parties promises to provide what they want (as I am). They then vote so that the party they would most like to see out rather than the one they want to see in gets ejected. Consider this: a seat is held by Labour and the Tories stand no chance of winning it. The LibDems, however, came a close second last time. Tory voters wishing to see Labour removed from office might vote LibDem to deprive Labour of the seat.
Many also consider that, in a great number of constituencies, a vote for one of the two major parties is “wasted” unless you happen to live in a “marginal”.
There is a principle misunderstanding shared by the electorate when voting in UK general elections (and one which all the parties go out of their way to encourage). That is that voters are not (or should not be) voting to elect a party, or a government or a Prime Minister. They are voting to elect an MP to best represent their interests at Westminster. Only after they have all been elected should the question of who the Queen invites to form a government be raised.
Of course the parties do not want this. They want voters to vote for “them” regardless of the candidate and indeed that is precisely what almost all of us do. This is because we are presented with two or three packages of measures which the parties say they will enact if elected to power (promises which they rarely fulfil). In 99% of constituencies the individual candidates’ qualities and proposals– and more often than not their identities – are irrelevant.
Only if party politics are abandoned and we return to individual MPs undertaking to canvass their constituents on individual issues and voting according to their wishes will this situation change. And there is as much chance of that as there is of the UK leaving the EU,