67.23% of all votes cast. Turnout was 64.62%, so 43.2% of all registered voters [1]. As fraction of total population, it was 56.23 million in 1975 [2] and the total number of Yes votes was 17,378,581, so 30.91%.

[1] https://en.wikipedia.org/wiki/United_Kingdom_European_Commun...

[2] https://en.wikipedia.org/wiki/Demography_of_the_United_Kingd...