I'd either HUCA, or tell them to refer to the award chart. I ran into this on my most recent trip to LHR on both directions. Y fare on AA metal going across, B fare coming back on US metal.
Many agents aren't used to working with Y/B fares, so they just rattle off the usual "miles and copay" line as these are relatively infrequent. Politely pointing out "It's a Y/B fare, there should be no copay" usually snaps them back with a "Oh yes, you're right. I'm sorry about that" and life goes on.
Keep in mind coming back from LHR that you have to pay the UK's premium departure tax, and that's collected at the time the upgrade's issued. Mine cost $103, and that's separate from the copay issue. There's no tax going TO LHR.