Maybe the government should collect the taxes at the gate or at the check in like in other countries. In Japan you have to buy an apd "ticket" from a machine which you then put into a turnstyle just like at a train station to allow you to pass into departures. After the initial outlay for infrastructure the system pretty much runs itself, consumables, like tickets and ink has to be replaced.
The benefit of this is that the airlines can display what looks like a far cheaper price and a lot of extra impulse bookings may be made. You think you are getting a bargain until you get to the airport, then it looks like the airport is the baddy and not the airline. Airlines would have to employ less accountancy staff to deal with all the paperwork in transferring the millions to the exchequer. ?
Bad idea. I used to avoid Ryanair like the plague, but I actually found them more comfortable than expensive flights with TOM or TCX. Their existence of '£5.99' flights which round up to a total of £23 with added fees would just be made more expensive. That's because the airport subsidies cover the cost of some of the APD for some of FR's destinations, if it's paid direct by the customer then it remains a lose/lose for FR and their customers.