To take advantage 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 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 the station to the airport.
Going to the airport by train is great, you can take a nap, read a book (even at night), play a game, or just stare out of the window... The other restriction to buying these cheap train tickets from northampton station to gatwick airport is that there must be at least two adults travelling together. But another great benefit is that children go free. All children under 5 go free, and then one child over 5 but under 16 per grown up travelling on the train to the airport.
be sure to leave plenty of time to get to the airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with spare time at the airport you can always read a book or play travel scrabble again! As you have bought yr ticket in advance, you don't need to fret about the opening hours of northampton station.
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