It must be real
A man in a traffic jam on the M4 saw it (he was on the BBC). The fact that the M4 is slightly more than a kilometre away from the runway, and where he reported it (Harmondsworth) it runs in a bit of a cutting, suggests that it was a) either a really BIG drone over the runway or b) he has really GOOD eyesight. Can't see any other possibilities. The lights he saw were stationary, so couldn't possibly have been on a tower or anything like that - which at least means that it will have been easy to photograph.