Luton Borough Council don't have a residents parking scheme in the area very close to where you can get a free bus from the terminal.
Hmm, nor does Solihull MBC in Hampton in Arden (3 mins on train), don't think they do around Marston Green either. Plenty of parking shuttle buses milling around there, but not sure if security would just let you walk in, probably not!
Concession pass holders wouldn't pay for the train anyway.