I have two DONE5 tickets booked with cash ex-CPT for me and a friend. The itineraries are slightly different, we are on the same flights for 13 out of 16 segments.

For the past 20 days I've tried to change the date of the first flights to travel one day earlier.

About 10 calls to BA Gold Line, supervisors involved, they finally managed to reissue the tickets and they charged me 3500 ZAR (~165 GBP) and my friend 280 ZAR (~13 GBP) for "difference in taxes".

The taxes difference on the original tickets were about 200 ZAR. Looking at the fare calculation I can see:

mine: CPT BA LON AA LAX AA ANC AA PHX
my friend: CPT BA X/LON AA LAX AA ANC AA PHX

I think the X/LON (we both transit LHR without stopping) is the culprit why I wasn't charged the lower tax.

BA site doesn't show me the taxes breakdown so I cannot confirm the UK departure tax is involved. Anyone has experience how to handle this and how to claim the taxes back?