SHOCKING CUSTOMER SERVICE

I booked a flight from Durban to CT 8 months ago..flight BA 6300 8th Dec 2017. We arrived an hour before take off to be told that the flight is FULL??!! The manager said it was just bad luck and we had to get on the next flight. She said that too many tickets had been sold?!!? She agreed that we had done everything correctly and were on time and that it was BA s fault!!THAT BA, is unacceptable behaviour!! How can you treat a customer like that!!??!!