That was in the initial stages, when there was little, if any, window smashing going on - if you look at the pictures you'll see that a few protestors got in initially (through the door, and then through one broken window) but that then other windows were broken from the inside as well as from the outside.
It all got an awful lot worse later and anyone working in the reception area (and offices up on about the fourth floor (where windows were also smashed from the inside, fortunately not causing large panes of glass to fall onto the people below, I suspect more by luck than judgement) would have been absolutely terrified!
There were plenty where I was working who would have loved the chance to confront the teeny boppers