Its all very well saying BA should have booked the passengers on other services, but which ones are they? The schedule departure was 21:20. After the tech problem had been Identified and the passengers offloaded it would be 22:20 at the earliest. What flights are there from MCO-LON after that? So sending the pax via DFW, ORD etc BA could not get the pax there in time for the trans atlantic flights that night. BA assumed the aircraft would be fixed next day, so much easier to put the pax in hotels and send them next evening. Of course it all went wrong after that, but at the time it was not forseen.
I quite agree that BA should put pax on other carriers when a tech problem causes a flight cancellation, but that can only be done if there are other flights available. The problem with late night departures from the USA is there are very few daytime flights to Europe to transfer the pax to.. And that late at night there are not many domestics to other hubs either. None of this excuses the disgraceful behaviour of BA in ignoring their EU261 obligations when the second tech issue arose and the aircraft diverted to JFK.