Who are these barons?

They are those in control of the unions (and I don't mean their members) who control how the unions' votes are cast.

To quote Laura Parker from Labour for a New Democracy:

“Rank-and-file trade unionists play a key role in our campaign, but despite this most unions do not yet back reform. The truth is, if the leadership had engaged with this unifying policy as intensively as they pushed their own proposed rule changes, PR would now be Labour policy.”

Above quote from The Guardian