In my early days as an assistant at West Drayton, BEA still operated Argosies on a night time freight run to Paris. The outbound clearance was usually something like 'Seaford One Departure climb to FL70 to be level by the FIR boundary'. Needless to say they rarely managed this, in fact you were lucky if they made 4000ft by the south coast! The only saving grace was the Air France freighter was a Breguet Deux Ponts, and that had a rate of climb even worse!