'Paddy Zulu' was a cunning plan to get an ancient airframe modified whilst on lease! IIRC one of the mods was to the MED's that ensured the girt bar was disconnected when opening the door from the outside if it had been left in auto! The trailing edge flaps were re-rigged to improve crz performance. Changes made to the 'boat fairing' mechanisms to do away with the fixed tracks. I think BA also installed the -7 engines and with that went the throttle bar. Another mod was to fit floor seat tracks in a lot of the cabin the previous occupants of the space having turned into aluminium oxide!
Mind you it was a looong time ago!