There is already a screen of bullet proof glass across the public gallery BUT (and this really is the at the heart of most security preparations in Britain) the area where the protestors were sitting was not screened off because they were in an area for invited guests of MP's and Lords and so could be trusted not to do anything so unseemly. The upper echelons of the civil and security services seem to have difficulty in adapting their threat mindset to anyone who won't play 'by-the-rules'. It just isn't cricket you know...