Unfortunately, the 100 series Avon has a history of power output incidents that Rolls-Royce were unable to fully explain, even when they were fully responsible for the engine in Service use. The AAIB report quotes this fact. Additionally, the Avon fuel control system includes several subsystems, other than the bleed-air system, that interact and effect engine performance. I suspect that no-one today can prove exactly if that engine was performing correctly on the day of the accident, apart from the stated parameters in the report. However, there does appear to be some conjecture bias against the pilots actions, rather than the machine.
As always, I try to limit my comments to facts.