I may be wrong (I was only 10 in 1967!), but wasn't the car park opposite the teminal the only parking available. Although I vaguely remember that one of the hangars round the back was used for undercover parking at some point.
That was Luton Undercover Airport Parking AKA Central Car Storage. They are still going and have a compound by the railway track in Kimpton Road.
They were in one of three T2 WW2 hangers that can been seen at the top of the page on the left of the photo and were set back from the road after being separated from the apron. Two of those hangars are still in place