Yes, agreed, but you've missed the point - the people who use the Park and Ride sites have already driven! So if they've already got in their cars and driven to Brighton then the environmental damage is already done. Forcing them onto buses rather than letting them walk simply compounds the damage and cannot be described as 'sustainable'.Because a bus transports dozens of people by itself, a train hundreds of people. To get the same amount of people there by driving results in far more carbon emissions than using a hybrid bus or the trains.
Busses and trains are far less polluting than transporting that many people by car.