That sign has been there since February!
http://www.flyertalk.com/forum/briti...-allowace.html
Some time last year BA put out signs and sent emails out saying that provided your bags are inside the HBO limit then they will check them for free. On busy flights you will get a pro-active email and text message inviting you to check bags in for free (which I think can then be bigger than your HBO allowance, but I'm not sure about that).