Hi vSFU.
I tried to post a similar thread a couple of days ago but got no replies (maybe it's just me!!). I too have had problems with this but not just in taxes. I have reproduced the thread here:

Can someone explain this (as BA can't):

I just bought a ticket on-line booked FCO-LHR-IAH as "business flexible". My meeting has changed days and when I went to MMB to 'change date and time of ticket' I was allowed to change my date of travel - for 2000 euros!! When I rang BA in London, they said there were no endorsements and the ticket was fully flexible and changed it there and then for free.

I have sinced tried to change a 'fully flex' F class ticket from GVA-JFK and have received prices ranging from 1.8 euros to 1600 euros. It is apparently something to do with the computer not being able to link the GVA-LHR sector with the LHR-JFK sector and therefore imposing additional charges. I still don't follow this as the GVA-LHR is obviously C class (as journey booked in F) and therefore should be fully flexible with no endorsements.