Bit late to add to this one but I think you've all missed the point!
QFE = pressure at aerodrome used for landing & operation in "the circuit"
QFF = sea-level equivalent of above (my interpretation)
QNH = sea-level pressure for a given area or Altimeter Setting Region (ASR) eg Barnsley in NW England. this is the lowest pressure in said region. All en-route aircraft using this value will be singing from the same hymn sheet if they encounter one another thus avoiding the dangers of different altimeter settings.
None have anything to do with ISA they are current values.
QFF isn't something I've encountered in modern-day aviation. Having recently gained my PPL I have a reasonable grasp of the subject
