To take advantage of the best prices you will need to prove you are not a business traveller, on the way to gatwick airport, so you will need to give either yr flight details or your holiday booking reference. I don't think you will need to provide this when you arrive at harpenden station but you may be asked to provide them on yr journey from the station to gatwick airport.
Going to the airport by train is great, you can sleep, read (even at night), play travel scrabble, or just gaze 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 kidover five but under 16 per grown up 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 harpenden station before you go. If you end up with spare time at gatwick airport you can always read or play a game again! As you have bought your train ticket in advance, you don't need to fret about the opening hours of harpenden station.
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