A gun was involved in the incident.
An attack on a soldier (who is trained to use firearms) and hsi subsequent public murder was the incident.
It took officers 30 minutes to respond, according to some of the people who witnessed the attack. During that time, the attackers could have take the opportunity to kill multiple others if they had chosen to, and the citizens would not have been able to meet deadly force equally.
If the police are not there to defend the citizens, why take away their ability to defend themselves?
One item to note - I have been to England and London before the gun ban, and even then, guns were not as prevalent there, as we see them here. They just aren't a "gun society", so in all honesty, I doubt any of the civilians would have been carrying a gun, even if it was legal (and easy).