Those proposing he's unelectable generally believe that a leftist government is a thing of the past in Britain – the last leftist government ended in 1979 following the Winter of Discontent, a series of widespread strikes in many industries reacting to poor economic governance. Margaret Thatchers' Conservative government was elected directly as a response to the poor performance of the previous Labour government.

Labour remained in opposition for 18 years, until Tony Blair won the 1997 election with his New Labour campaign – a restructuring of the Labour party to appeal to the centrists, inspired by the Clinton campaign in the US.

Many in the UK–including Tony Blair and many of the elected Labour ministers–argue a Labour that doesn't appeal to the centre can't win. I would argue that since Labour lost its large Scottish base to the Scottish National Party (SNP) in the 2015 election, a centrist Labour government can't win.

A shift to the left will give the much ignored left-learning portions of the electorate something to vote for again, hopefully increasing youth turnout and winning back some of the Scottish voters who went with the more left-learning SNP; and may well allow Labour to ally with the SNP to form a coalition government.

Either way, I don't see Labour winning again in its current incarnation; unless the SNP royally screws up and fails to keep its current voters. Something has to change in Labour.