OK so various taxes, fees and surcharges have codes assigned to them. GB - APD, YQ = BA profit fee etc eyc
However these codes aren't necessarily a standard list so one country /airline can give a two letter code to a charge that gets translated to some other (often previous) meaning that dosen't always reflect the current version.
So it is perfectly possible that ZF was used by BA eons ago for one purpose but that no longer reflects it's current usage.