I think the agent is correct, but with one minor caveat. The fare codes are correct. For an Iberia-operated flight the booking classes eligible for UuA are B and Y. I also think you have to book under the IB flight number in order to UuA. The BA codeshare would not be eligible. That's based on what's stated here: https://www.flyertalk.com/forum/32910040-post7.html