I also think the obscene prices charged on board, has an influence on pre-loading at airport bars. It must be 90%, or more, profit on flights to non-EU destinations as the airlines charge full VAT / duty inclusive prices but don't pay a penny to the exchequer. Bar-stewards.
Was at BRS the other day and got a pint of Butcome at the airport bar. On the flight to GVA a 33ml crap lager cost more (and they paid zero VAT/Duty). Bar-stewards.