Promised flights that don't happen
In the past few months there've been many unusual aircraft at LHR & LGW, among others. For instance, from Vietnam we've seen Bamboo Airways, Air Vietnam A350s and B-787 10. These flights are predicted about a week in advance on spotters' forums.
Some predictions, however, don't come true. A Cubana IL-96 at Gatwick this week is the most recent I've noticed.
What's happening? I would guess that if an airline goes as far as booking a slot at Gatwick it has got a planeful of people or goods that have been paid for; aircraft, crew, all the necessary permissions. And if one planeful can be put on other flights, what's behind China Southern having a daily 777 flight to LHR that never happened?