We are lucky.
Use a drone to drop a small explosive at the O2 arena after a show. Dump the drone on a random roof...
4 days after that, hit Liecester Square.
London would come to a standstill, civilians would be afraid to visit open public areas, just as we saw in Mosul. The explosions wouldn’t even need to be large to have this effect.
However, the effect it would have would be out of proportion to the casualties - it would demonstrate the inability for the civil powers to protect the population.
After Syria, the attempt on Maduro’s life and Gatwick, the potential for use of drones by malicious actors is patently clear. END