There have been several opportunities for a re-alignment of the Left since 1918 : at the time of the minority Labour government in 1929-31, during the Lib-Lab pact of 1977-79, at the time of the Jenkins report in 1997, and in 2010, at the start of the Con/LibDem coalition. The cause for such a re-alignment would have been an agreement on electoral reform. This might have happened in 1997, had a significant body of opinion in the Labour Party agreed with Blair on electoral reform. As Jenkins said in his characteristic way in his report, the Labour Party, having been given a 'luscious cornucopia of electoral fruit' at the election of 1997, was unlikely to endorse any form of PR. And so it proved.
The Labour Party's attitude to first-past-the-post throughout the 20th century (although its attitude may be changing rather belatedly now) can be likened to travellers in a desert who experience hallucinations of an oasis. There is one difference: that every so often the Labour party finds the electoral oasis, or cornucopia, which convinces it that first-past-the-post is not s rotten system, and is not biased against it. There is also the unexamined assumption that the United Kingdom's political system and culture are inimical to minority governments and coalitions, when, in fact, this description would fit most British governments between 1880 and 1945. Through a deadly combination of historical amnesia and self-delusion, Labour has supported first-past-the-post throughout most of its existence.