I had a similar experience JFK-LHR last winter. Checked in, flight wide open both classes. At gate the first agent assigned me a J-Class seat, and the woman with him said "you can't do that, he's a j/s. Only allowed to assign coach". A bit of a bummer, but the Purser moved me up as soon as boarding was done anyways. No biggy, I certainly appreciate the free ride but also wonder if this is a new policy (FWIW my buddies at CO tell me that IS their policy, so I avoid them across the pond.... CO coach is pretty bad).