The test will be that as far as the police officer who did the shooting is concerned, was the officer's belief (that there was an imminent threat of an explosion from a suicide bomber) an honestly held belief, even if mistaken? It clearly was mistaken.
The issue of reasonableness in the law of self-defence concerns the level of force the accused used. The force used must be proportional to the believed threat - so Tony Martin, who shot a burglar in the back whilst he was fleeing, was not able to use the defence of self-defence / prevention of crime as the judge ruled that the level of force used was not reasonable (so the jury did not get to hear the self-defence plea).
It seems to me that if in the circumstances as the police believed them to be (will say they believed them to be), they thought killing was necessary, it makes no sense to question the amount of force used in the killing (one shot, five shots, eight shots).
Many contributors to this threat have pointed to many anomalies in the police account that we currently have. In the light of those, it seems that there are some grounds for wondering whether the police did have an honest belief that they were following a suicide bomber, as opposed to being paniced when the victim did a runner at a tube station and the police then shot him to stop him.