The Fulham West by-election 1938 was won for Labour by Edith Summerskill, taking the seat from the Conservatives by 52%-48% in the face of widespread predictions by pundits that the Tories would hold on.
Why mention this? Because the contest also saw the first by-election opinion poll in Britain, and it called the result correctly:
From Opinion Polls: History, Theory and Practice by Nick Moon
Later, in 1945, Pathe produced this short film about her political career which was one of the most high profile in British politics at the time: