If you are a plus sized traveler DO NOT BOOK BA Premium Economy. The seats are 18.5 inches wide, 1 inch more than a normal coach seat. True there is more leg room but if you can't sit comfortably in the seat who cares. The design of the seat is a hard shell rather than a typical airline seat.
My spouse who is handicapped was told by reservations (more than once) that the arm rest on the aisle side of her seat would fold up to make it easier for her to enter / exit the seat and give her a little more hip room during the flight. She could not physically sit down into the seat. In short they lied or did not know their own aircraft configurations on on the 747 Version 1 & 2 aircraft from SFO to LHR.
I bought World Traveler Plus - Premium Economy to give us a little more comfort but because of the design of the seats the opposite was true. She could not even sit all the way into the seat and was horribly uncomfortable the entire 10 hour flight.
Reservations also told us multiple times that the aisle side arm rest would fold up on an Airbus A321Economy seat but that was not accurate either in that only armrests that will fold up are if you are assigned a seat that is behind the window exit. According to the head cabin attendant on our FCO LHR flight that is their normal intra Europe configuration and he was baffled why reservations did not know that.
We normally fold up the arm rest between our seats to give her a little more room in United Premium Economy or Alaska Economy. Due to the hard shell design of the BA seat that was not possible. We were actually more comfortable flying home in World Traveler (Economy) than using the BA World Traveler Plus (Premium Economy). Also BA refused to refund any of the money I had paid for World Traveler Plus when we took the return trip downgrade.
If you are a big person avoid BA World Traveler Plus. Tall is fine but if you are big and tall forget BA World Traveler Plus.
The only bright spot of this experience is the cabin crews on our flights. They were helpful and apologetic about the problems. Stan the crew leader from Rome to LHR helped her connect at LHR by insuring she got handicapped service. The Service Director, Matthew, on the SFO LHR flight even created a case to customer relations which was ignored in spite of two follow up phone calls and two follow up emails over a 1 week period.
Buyer beware of BA's Premium Economy.