In my many years with BA, I have never had any occasions when I've come across a BA agent telling a passenger that they were late as an excuse for oversales. One of the things we are trained on, is to be honest with the passengers in all situations. Given that you were in CW, even if we were oversold in this cabin, then at worse you would have been downgraded, and not to a situation where you would not be accepted for travel.

I'm a bit surprised that after being informed in the lounge that check-in closes at -45, you did not seek to speak to a duty manager to have the situation looked into at that stage.

As for the PHL flight, if the flight was not available for sale, then the agent couldn't have booked you onto it and depending on the booking figure on the flight, then they would know best. The fact that there were seats free on departure could have been down to a number of factors, including late transfer passengers who were previously checked in. Just out of curiosity, how do you know that it left with seats??