To take benefit of the best prices you will need to prove you are a leisure traveller, en route to gatwick airport, so you will need to provide either yr flight numbers or yr 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 travels from harpenden station to the airport.
Going to the airport on the train is great, you can take a nap, read (even at night), play a game, or just gaze out of the window... The other restriction to buying these cheap tickets from the station to gatwick airport is that there must be at least two adults travelling together. But another great advantage is that kids go free. All children under 5 go free, and then one child over 5 but under sixteen per adult travelling by train to gatwick airport.
make sure you leave plenty of time to get to gatwick airport early enough to eat, shop and so on, so check the times of trains from harpenden station before you go. If you end up with time to kill at gatwick airport you can always read or play a game again! As you have bought yr train ticket in advance, you don't need to fret about the opening times of harpenden station.
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