Booked premium economy for roundtrip JFK to LHR.
Outbound Flight was American Airlines but return from London was a code share with British Airways.
I selected seats in advance on American but British Airways would not allow seat selection without an extra fee even though it was premium economy price.

Even if I paid to select seats they wanted to put put me in regular economy cabin.
What is the benefit of paying for premium economy on a code share flight with British Airway if you can't select seats in advance and don't get a better seat?