This could not happen. It is a fake story. The law in the UK is very clear, so this did not happen.
Here right from gov.uk https://www.gov.uk/find-out-if-i-can-buy-or-carry-a-knife
But in truth, what a horrid and sad story, so many witnesses and no one did a thing to stop the attack. I guess its just a cultural difference.
Your last paragraph is scary because that cultural difference is quickly fading away. Kids in school are not taught to step in when they see something that is wrong, they are told they should go find a teacher. This follows them in to adulthood where they see a woman being attacked and instead of stepping in to stop it they call for help and stand by until the police finally arrive.
It kills me to know what is happening to the minds of the children in this country.