From two news reports :
The only two airlines that ply the London-Harare route are British Airways and Air Zimbabwe. British Airways has denied any of its aircraft were involved.
BA don't fly LON-HRE. They only operate into ZW as a feeder from JNB.
David Mwenga, the Air Zimbabwe Europe and USA general manager said: "I am concerned that this incident led to many, many calls to us. People were frightened. No actual plane was involved, but there was a scenario involving a Boeing 767 plane that had been hijacked and forced down at Harare airport."
Highly unlikely ..... hijacking a plane to leave Zim seems a more plausible scenario!