I would repeat the concerns about emergency vehicles getting access to the front of the building when it's choked with private cars, not to mention the aviation security issues of unknown vehicles "abandoned" right outside the door.
Barriers are the best way to reduce the risk of these issues becoming something more serious.
For heaven's sake people, the place is short of land. What else is it to do to regulate access? There are enough other ways of getting there. Do you expect to drive into Manchester to go to Kendals or Selfridges, and park outside, on Deansgate or whatever road they're on, for free?