An Airspeed Ambassador, registered G-AMAD, was flying a cargo of 8 horses from Deauville to London. When approaching the runway 28R threshold at London Airport, the left wing suddenly dropped. The wingtip, followed by the left main gear wheels touched the grass to the left of the runway. The crew tried to increase power to go around, but the aircraft flew on with an increasing bank angle. The plane struck two parked BEA HS-121 Trident aircraft, burst into flames, rolled on its back and came to rest against the ground floor of the terminal building. Trident G-ARPT (msn 2121) was damaged beyond repair; G-ARPI had its fin and tailplane destroyed but was repaired. PROBABLE CAUSE: Failure of the port flap operating rod due to fatigue, permitting the port flaps to retract. This resulted in a rolling moment to port which could not be controlled.