It's also my experience of said airline that their aircraft are often excessively hot.
At the risk of repeating... It's not exclusive to BA.
Unfortunately, passengers are no longer content to read a book or watch a movie shown on a cabin wall. Now, everyone wants their own video screens and movie/game machine, so basically, a large electronic box has to be put under each group of 2~3 seats to control/process the video. These probably generate as much heat as an average laptop computer. Even the LCD screen generates heat because of the backlighting.
The overhead air vents can be a relief. The air source for these on the 747-400 is simply the air in the roof (above the cabin ceiling), which often can be quite hot on the ground. However, the air is moving, so cools the skin through evaporation. Yes, many Boeings do have vents (including 747-400's), but it is a customer option.