I don't know what the safety reasons for the closure are but I imagine it relates, at least in part, to the fireworks' "fall out zone" during the display. The current internal route inside Chiswick Business Park from Bollo Lane goes right past Building 9 where the fireworks are set up on the roof. Add to that the relatively narrow staircase you have to use inside the Park on that route plus the large crowd involved and you might want to avoid keeping open an entrance with the potential for a large last minute rush of people? Anyway, apologies again if I've misunderstood your point.
And that crowd at the very start of last night's display:
