GUC datapoint as DM for DTW-LHR round trip:

1. Purchased economy (using miles).
2. Used GUCs to upgrade to PS at time of booking.
3. Cleared D1 waitlist for outbound flight at gate.
4. Did not clear D1 waitlist for return (despite plenty of D1 seats available at T-24 hours). Also had difficulty checking in.
5. Called Diamond line to investigate and had to pay the UK Air Passenger Duty ($185 pp) in order to be upgraded to D1 (which I did).

Overall, I was certainly pleased both legs cleared, but was surprised about the APD fees -- you learn something new every day!