"they will admit privately that it was a complete cock-up"
It was a cock-up from the moment that the police decided that they had justification for Operation Kratos, in which SO19 firearms officers were permitted to make what are euphemistically called critical shots, ie shots to the head. They're not trained to do it, nor are they really equipped to do it. The justification was to stop the detonation of explosives carried by the suspects, but the atmosphere must have become progressively more heated. Hollow Point ammunition was issued, because there was more concern about what might happen to people stood behind the target. Officers were regularly told that explosives might detonate at any time, so they had to be fast with their first shot.
The end result was a man being shot repeatedly in the head at point blank range by officers who essentially kept firing until their guns were empty. They cannot rationally have believed he was carrying viable explosives: the 7/7 and 21/7 bombings and bombing attempts had needed large rucksacks because the explosives the terrorists had manufactured were bulky and inefficient; JCdM was by contrast wearing summer clothes and carrying nothing.
But the whole febrile air meant that the police, who were not trained nor given appropriate doctrine and rules of engagement, believed that they had to stop him by any means necessary. In which case, you have to ask, why the hell did they let him enter the station? Even with JHP ammunition, the risks to bystanders were considerable, and the effect of a detonation on a train, had he been actually carrying a bomb, would have been orders of magnitude worse than a detonation in the ticket hall or the street. So even had he been a terrorist carrying a viable device, it would still have been a total cock-up.
The atmosphere amongst the police that day was that they were the last line of defence against a well-resourced, competent opponent. In fact, had the threat been as serious as was made out, the police weren't suitable people. But given the police were being used, the tactics used were entirely inappropriate for the skills available. The individual officers are guilty of nothing but carrying out orders they must have known were inadequate. Collectively, however, the police failed, and the buck should have stopped at the Gold Commander.