I'm not familiar with the exact locations and this may have changed - but the display ads (banners etc) were contracted out at Gatwick on a long term deal. So BA would have had to have bought the space - I imagine they agreed significant marketing support from GAL as part of the move though to fund it.

So over time it may "de-BA" a bit as these media buys run out. From what I've seen at T5 it's a favourite activity of Virgin and Emirates to buy them to show BA passengers what they are missing!