Hmm...
People need to get from their home near Luton to Alicante. They can either pay to park, or pay to get dropped off. The airport love it.
People need to get from their home near Luton to London. They can either pay to park at the railway station (which let's not forget during all the years of Standard Open fares being capped, ancillary costs like parking were not) and catch the train, or they can be dropped off for free and catch the train. However, they have a third option of driving all the way and not paying anything at all to the railway.
Driving all the way is not really an alternative option for the vast majority of air travel. Paying to not walk to the airport, and paying for overpriced bottles of water which you have to put in a bin, and then paying for more overpriced bottles of water the other side of security is all part of the fun money-go-round of air travel.
Driving all the way is an option for a considerable amount of rail travel, and I would say that some railway station parking charges are already pushing their luck and quite possibly actually being a deterrent to rail travel.
To add a further deterrent with a drop off charge, pushing people to get a lift all the way, or Uber etc. would be suicidal.
But then, anyone who agrees with me doesn't matter. It'll be some Civil Servant or CEO type 'exploring' extra revenue (disregarding driving any away) that will decide.