I assume that most routine Northolt traffic (VIP or otherwise) either fits seamlessly into the Heathrow pattern or goes round it. Only conjecture, but much more likely that a “secret asset” developed an in flight emergency and the RAF responded to a Mayday. I imagine that had a civilian “asset” required an emergency priority approach to Heathrow, similar delays to other civilian traffic would have occurred, but we would all understand why and the topic would have gone away by now. This is a common fault of the security services – to let everyone know that there is a secret which you are not sharing. Nothing arouses public curiosity more.