Nivsy, I'm certainly no apologist for BA either but I do see the logic in boarding you on the aircraft despite the slot. All airlines would do the same. Improvements in slots are very common and in a lot of cases are actually lifted if you are in a position to tell ATC that you are fully boarded with doors closed and ready to go. There are days however when this dosen't happen and you obviously were in that situation yesterday.
If it's any consolation, it dosen't make our job any easier having you on board an aircraft for two hours either. Tempers can fray and whilst we are protected from it all in the flight deck it's normally the cabin crew who bear the brunt of it. What you have to remember is that it also means I'll be home to my family two or three hours late also, and this is certainly not something one would expect to set out to do at the start of a day.