BA - Very Disappointing service

On the 12th of September 2019, my wife and I were booked to fly from Johannesburg to Durban on the 15:50 flight. We were early at the airport, around 11.20. There was a 12.45 flight which we noted from the information board. We went to the BA ticket sales counter and requested the change in flight. The agent (Karabo) assisting us asked whether we had booked this through the call center. I advised that this had been booked online, nothing fancy just a straightforward booking, with an Avis rental car. She explained that the system is not allowing her to make the change and the helpdesk needs to be contacted. I asked her to do what was required and get us moving along. She went to the backoffice and from my understanding was making contact with them. About 25 minutes later, the supervisor emerged to advise she was still busy on the phone. I explained to him that all that I wanted to do was change my flight, pay the difference and move along. Anyway, long story short, he tried calling the supervisor (Serrod Harris) of the call center, no joy. I asked him to call his manager to see if there could be a resolution to what I see as an insignificant issue, no success. The manager Mags Reddy (from what I was told), had the audacity to advise Serrod that we be told there is nothing that they could do. My wife by this time were quite tired, now standing at the counter for almost an hour, with no resolution, not from the agent, the supervisor or the so called manager. Very disappointing indeed. We went over to the Mango counter and purchased 2 tickets for the 1.50 flight and flew to Durban. We did not use the BA tickets. Considering the ******* service, I would certainly not encourage anyone to fly BA. If a simple ticket change cannot be managed quickly and efficiently (if at all), I would be most concerned about the way they manage the rest of their business.