Sounds like a load of cobblers to me. BA regularly have diversions into LTN and STN when there's disruption at LHR and sometimes the company decides to offload the pax there and coach them to LHR. My guess is that this stems from a desire to keep the pax on so you don't have to open the holds and get their bags out (departing with the bags on board would obviously be unacceptable). Barring some local peculiarities with customs I can't think of any reason why the pax couldn't get off.