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 your flight numbers or your holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to give them on your journey from romford 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 a game, or just gaze out of the window... The other caveat to buying these cheap train tickets from the station to the airport is that there must be at least two grown ups travelling together. But another great advantage is that kids go free. All kids under 5 go free, and then one child over five but under sixteen per grown up travelling on the train to luton airport.
make sure you leave plenty of time to get to luton airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you end up with time to kill at the airport you can always read a book or play a game again! As you have bought yr train ticket in advance, you don't need to fret about the opening hours of the station.
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