At the Labour Party conference of 26/04/74, only seven of forty-six trade unions present supported EC membership, and the Labour membership rejected continuing membership of the EC by a margin of almost 2:1.
Errr that was 1974. The clue is in the date. Only two (very small) unions supported Brexit - the RMT (who are in a unique position for many reasons) and the Bakers and Allied Workers Union.
It's a great shame that we didn't get to hear from more union voices during the Brexit negotiations - especially e.g. the Unite officers who are in charge of collective bargaining on the union side in the car industry, or, say, Dave Prentis of UNISON.
The BBC has a lot to answer for with the disproportionate coverage it gave to the likes of Farage et al, while failing to listen to rational voices on the other side.