How can one establish what the duty element of a flight is?

For example, look at this comparison:

EasyJet LGW -> Marseille (Return)

Adult Fare GBP 24.12
Adult Tax and Service Fee GBP 33.50
Total:  GBP  57.62

BA LGW -> Marseille (Return)

Adult Fare GBP 14.00
Adult Tax and Service Fee GBP 51.10
Total:  GBP  65.10


It would seem BA's ticket is actually cheaper that EasyJet, but they ask for
52% more on the Tax & Service.


TIA
Jerry