Too early to say. If the Tories mess things up regarding Brexit and/or Corbyn is ousted before the election and replaced with a non-radical person who is acceptable to the voters they lost in England, Labour could win in 2020.
Comparing Labour with the GOP is an apt analogy.
Both parties have bases that are radical enough to make them lose elections but large enough to keep them competitive. While it appears hopeless for each party, they could get their acts together/their opponents could mess up at anytime.
