parties can’t win elections in the UK anymore from a leftwing platform

How would we know? Which party with more than a couple of percentage points of the vote has actually run an even slightly leftwing campaign since Blair and Brown? The only one I can think of is Corbyn, and he was sabotaged by his own MPs and the party apparatchiks who would (somewhat like the article) rather lose than allow anyone with even vaguely leftist policies to win.