One data point to add: me and the girlfriend flying EWR-LHR-EWR-BOS. Booked premium plus and was ticketed into B fare on the EWR-BOS leg. PN and PZ space available. I'm 1K, she's Plat. Saw the confirm instant upgrade button on the manage reservation page but when I clicked it it just took me back to the UA homepage (as many have reported). Used the chat feature 3 times to request an "instant" upgrade but the agents had no idea what I was talking about. Called the 1K line and the agent cleared the upgrade for both of us into PN after speaking with a supervisor.

And with PZ space available at booking on both of the transatlantic legs, it does feel like a quite a nice benefit of 1K to book a trip to London in Polaris for one grand/person.