Worst experience ever

Yesterday I was supposed to be on flight BA6262 from Harare to Johannesburg and then later on and got there and they I told them I had checked in already and I showed them the boarding pass on my phone and they denied me entry saying that I should provide a hard copy and they only do manual check in at the Harare airport which was not mentioned on the terms and conditions of BA, I left and went to print and was back at 13. 10 and they said they have already closed and boarding had began. They redirected me to the ticket office and yet they knew that the ticket office was closed and apparently the same staff at the ticket office is the same staff that checks people in etc. According to the terms and conditions check in closes 45mins prior to scheduled departure ofwhich I had checked in in the morning and was still there before it had closed. I asked the lady to either move my connecting flight to a later time and she said it's not her job and walked away and since the ticket office was closed there was nothing I could do but go and buy a new direct flight ticket at SA Airlink for R3030. Since I had checked in I should have been allowed in because according to the terms and conditions one must be at the boarding gates 20mins prior to departure. I am not happy with the treatment that I received at Harare International Airport and something should be done