Had exactly the same experience with my other half who needed assistance at T5 following a knee injury. Wheelchair assistance didn't show, the CCR desk agent blamed her for some reason, and they frantically managed to get a buggy at the last minute. Luckily she had checked bags and they hadn't managed to offload her yet. She made the flight, which departed a few minutes late due to her.

The answer you will get on here is that BAA look after disability assistance and it's not BA's fault.