They are stuck into supporting their shrinking support base for ideological reasons.

Unions are strongly connected to both social democratic party (SDP). Unions and social democrats achieved many good things for several decades. But things have changed and they and their support base is trapped in the past and looking back to old good times. They are basically party for baby boomer generation.

I believe the Unions' thinking goes more like that they represent a constituency of traditional 9 'til 5 workers with steady income from one employer. Since such citizen are more likely to have Union membership, the Unions are fundamentally opposed to any social policy that would erode the basic motivation for majority of citizens to choose 9 'til 5 employment, like UBI. Remember that membership is everything to Unions.