UA have the IFE boxes under every 3rd or 4th seat (often but not always under the aisle seats.

Seems odd to me why IFE should affect seat WIDTH rather than legroom/pitch? Seat design may play some role here (I've never used BA economy long haul, but within Europe their coach seats do seem to "cocoon" you more than most (ie they curve upwards at the edges rather than just being flat), and the armrests seem wider than som other carriers. Maybe that's the explanation?