Do they just accept that to maintain the backing of the party in future elections that they have to accept voting along party lines forever?
Spot on post. I just read that article in the Guardian at almost the same time as reading the start of Robert Greene's book The Laws of Human Nature where he talks about rationality.
Politicians of the governing party on both sides of the Atlantic are focused on maintaining the ideology of their side irrespective of its effects on the lives of the vast majority of the people they serve. They are also fixed on maintaining their grip on power and will do whatever it takes to do so even if it harms democracy, hence attacks on free press, election procedures centered around voting and an independent judiciary. This irrationality means Trump and Johnson are doomed to failure and that its very difficult to change the minds of most of their supporters; they refuse to believe that they could be wrong because their egos won't let them.
I think a different election system in both countries (like some system of PR) would help along with the introduction of principles of psychology and philosophy into education systems so that ordinary people can begin to appreciate more how to make rational decisions based less on emotions and more on facts and evidence.