The airport is looking for a one ticket solution. It exists now except with a dotted line shown on maps many passengers only buy a ticket to Parkway and not the Airport. It is not so bad for passengers arriving by air as they will buy a ticket in the terminal for London or where ever that includes the bus link.
The airport are claiming the complete journey from terminal to London in under 30 minutes via the DART while in reality without the 4 non stop trains an hour that the new franchise rail agreement didn't give the airport, only a very few passengers will ever complete the journey in that time.