At the same time, we're not talking about amateur drone pilots here. Police forces train up dedicated drone pilots, and I'm sure BTP will be the same, and as they work on the railways all the time I'm sure they'll be acutely aware of the risks associated with flying above or around railway property. Not to say that a drone running out of battery and hitting a pantograph is impossible, but I imagine the risk is quite low.
Not to mention that in order to film what is underneath a pantograph (e.g. if they were filming people placing rocks on the rails) they don't need to fly directly over the pantograph either - even cheap drones have high definition wide angle lenses, so they could keep a safe distance from the lines themselves, using dedicated apps for wind, thermals, etc.