I ignored the UK and Ireland in my search, and I found absolutely nothing trying to cover the period from 1945 to approx. 2000 so I'm leaning towards, 'No, there never was a referendum on adopting English in the higher education sector'.
There were two referendums on higher education/research sector in Switzerland in the 1970's: one in 1973 on promoting research and one in 1978 promoting research and universities though their content is not listed. I did not check 1945 to 1969 nor 1981 onwards for Switzerland because this page says their tertiary education is primarily in German, French, or Italian.
Hungary had a referendum on higher education funding in 2008 which was one of the very few mentions of the higher education sector in post-war European direct democracy overall.
The Netherlands did not have such a referendum as they did not have any referendums between 1805 and 2005.
This is also not brought out in the "List of referendums" that I went through on WP between 1970 and 1980, nor on the national "Lists of Elections & Referendums" by country on WP (country-by-country).
Further, looking at this article on higher education in English none of the European countries are listed as having all higher education courses in English. This leads me to suspect this may have only been a plan or a bill of some other kind—but there's still no evidence to point to where it could have taken place.
Perhaps the closest (by a very loose definition) I found to this was a presidential order in Algeria from 2019 where French was replaced with English in the higher education sector.