Been doing a lot of reading about earlier British govts and elections over my Xmas break. Striking how often positions of great strength have crumbled due to unpredictable or external crises:

Devaluation in 1949 was a mortal blow to the Attlee govt, which had looked unbeatable before this.
Devaluation in 1967 similarly damaged the Wilson govt (despite Wilson, who was in Cabinet in 1949, making Herculean efforts to avert it)

Eden looked invincible until Suez; Macmillan looked unbeatable until Profumo.
Major’s unexpected triumph in 1992 was turned to ashes within a year by ERM crisis.

Months after Blair’s second landslide, the political agenda was completely transformed by 9/11.
Months after Brown’s accession, the political agenda was completely transformed by the global financial crisis

Within a year of Cameron’s surprise majority in 2015 a different kind of surprise - Brexit - had set the agenda for politics which we are still following

With much discussion of Johnson’s transformative plans, worth bearing in mind that many postwar PMs have spent much of most of their time on agendas which emerged externally and/or unexpectedly. The politics we now expect from the next 5 yrs may not be the politics we get.

TL;DR: “Events, dear boy, events” /ENDS/

Feel free to add your favourite examples of the British ship of state being blown off course in replies