G-XLEB did fly to Johannesburg later that day as per the screenshot below, must be for logistical reasons perhaps? I always find submitting a form via this link https://goo.gl/TfNP67 and then selecting either " Proof you didn’t travel with us – BA flights only" or "Confirmation that your BA flight was cancelled" while then requesting in the comment box for the exact details of the cancellation brings about results. Hope this helps