Why not just compare the time as indicated from FR 24 with that given on a website for the Flight Departures at the associated airfield?
Because, in the case of LHR at least, that could lead to huge confusion. I often look at the Heathrow Arrivals/Departures board to get an idea of when a particular aircraft might move and it is surprising how often the departure board will announce "Departed. Airborne at xx:xx" when the aircraft is obviously still at the gate preparing to move.