Well the range of the transmitter, battery life and how brave the operator is. As least there’s no trees to worry about and it could always land on the deck (aircraft carrier style).
It would also be difficult to know who to follow you may need multiple drones for different parts of the fleet, if I was there I would need my own at the back of the fleet but the battery would run out by the time I finished.
Out of interest have you any idea what altitude the last shots were taken from – I’m pretty sure the flight path into Manchester is not far from there as you see them coming in over Stockpot on a regular basis!!!!