Asleep at the wheel?
You can see why LU has had a sense of humour failure over this ... they've been caught asleep at the wheel. Again.
The video clearly shows one of their employees only a few metres away to the left of the barriers, who is oblivious of someone stopping at the top of the escalators holding a pair of skis, who is there long enough to check for the all-clear from his mates and clip himself into the skis before commencing the stunt.
Just goes to show that relying on CCTV for security means you are only able to helplessly watch on a monitor as events unfold. The kind of stuff you really want to prevent happening in the first place is usually over long before you can despatch someone from the control room.