Since the debit fee is included in the price, won't you pay that even if you pay with a credit card (which has it's own fee)? Or will they remove it before they add the other one on? (Fat chance)
Nice try OFT, but no cigar.
Advertised or headline prices should include all components. If there are different charges (for different credit cards, for example) they should include the most expensive one charged by that company.
Should any consumer be assessed anything more than the headline price on the payment page, there should be an automatic €1000 fine per passenger.
I'd make an exception for Government (not airport, even if owned by the government) imposed taxes - since these can vary by airport.