Following the recent incident at Gatwick Airport, Heathrow have this to say:
https://twitter.com/HeathrowAirport/status/1082695381379702784?s=19
We are responding to a drone sighting at Heathrow and are working closely with the Met Police to prevent any threat to operational safety. As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause.
https://twitter.com/HeathrowAirport/status/1082695381379702784?s=19