Aircraft left LHR for Lusaka 2 hours 30 after landing, the Luton diversion may just be a case of confused computers.
Luton maybe on the wrong side of London from BIG but how do you know they hadnt gone around at LHR. Anyway i suspect you'd be on the ground quicker at Luton from FL90 in BIG hold than you would at the more congested Gatwick. You'd also cause utter chaos going to Gatwick whilst ATC tried to descend you across the SE/E deps from LHR and LGW.