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