If they shouted police, I would probably stop, 'low till I was in that position I'm not sure what my gut would make me do.
Run down a street, maybe, but while run into a tube station, jump the ticket barriers, then onto a tube train? While all the time, there's 3-4 guys behind you shouting for you to stop? Seems a little excessive if you thought you were going to be mugged?!?
Someone posted on the BBC website, that with all the attacks, and then some guy pulling a gun, he was probably think it was another attack. My question is then, why run onto a tube train?