This is one of the most significant findings of the AAIB report. He had a "gate" at the top of the loop - a certain height at a certain airspeed, and hitting it should have been a go/no go, where he could roll erect and abort the loop. He failed to meet it by a considerable margin but continued the loop. They also found that the engine wasn't producing anything like full power on the way up. Whether there was a mechanical issue that put him in the wrong position is irrelevant, though. He would have noticed that he hadn't met his gate and aborted. Did he fail to check it? Were his instruments lying to him? Did he misread them? 
I've never read the report if i'm honest and can only go on my experience of working with fast jets and been sat in the back / side seat of fast jets, also watching the videos of that tragic day.
To me, I would have thought it would feel totally different in the seat of the pants if you fly over the top, not fall over the top like he did hence my comments above. It just looked wrong

I know the Avon 200 series was slow to spool up but who knows if the reduced thrust was down to slow engine OR the pilot not opening the throttle quick enough

Did the aircraft have a ADR fitted ?