Operational reasons - seriously

On the 14th dec 2017. Once again British airways decided to lie to me. So i book my flight way in advance and select my window seat . I even go check in online to secure that seat the day before , only to come find out that when boarding British airways decides to bump me to a middle seat with no explanation . Eventually I request a reason and this is there response“ operational reasons” seriously, how can a window seat assist a operational problem? To make it worse , the lady my window seat is loud on her phone having a “jol” thanks but no thanks! I’ll never travel with your airline again !