Trafalgar
I don’t think its CX on this occasion, it’s the UK government.
https://www.gov.uk/government/public...passenger-duty The current Air Passenger Duty charge is GBP150 in C/F and GBP75 in Y. the tax is paid on the class you travel in not the class you book in so if you have a Y upgradeable to C and you are upgraded to C or F CX has to pay the UK Government GBP150, there are other taxes to be paid as well such as the Airport user fees. there are no exemptions for staff travel.
A while ago I travelled on a free ticket long haul ex LHR where we had to pay taxes only. The total tax bill was over GBP600 for the two of us. It is not uncommon for me to travel Y to the US where the fare is less than the taxes.
The UK government takes the opinion that if you can afford to fly you can afford to pay taxes. Sadly numbers of passengers have risen since the tax was introduced proving the government right.