To take benefit of the best prices you will need to prove you are a leisure traveller, en route to the airport, so you will need to provide either yr flight numbers or yr holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to give them on your travels from feltham station to the airport.
Going to the airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap tickets from feltham station to luton airport is that there must be at least two grown ups travelling together. But another great benefit is that children go free. All children under 5 go free, and then one child over five but under sixteen per adult travelling by train to the airport.
be sure to leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from feltham station before you go. If you have time to kill at luton airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to fret about the opening hours of feltham station.
Or maybe you meant