I flew LHR-SEA and back last month. At the airport I asked about upgrades from Y to PE, and they said it would be £250 to PE, or £1000 to UC. I decided to take the PE upgrade, so the check in age t sent me off to a 🎫 desk to complete the transaction. Once I arrived there, the ticketing agent said "oh, actually I can do £750 to UC."
On the way back, at the check in desk they said it was too late to upgrade to PE as they hadn't loaded enough meals, but that I could upgrade to UC for $850.