To take advantage of the best prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to give either your flight details 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 west-hampstead station to newcastle airport.
Going to the airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, 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 adults travelling together. But another great advantage is that children go free. All children under 5 go free, and then one child over five but under sixteen per grown up travelling on the train to the airport.
be sure to leave enough time to get to newcastle airport early enough to eat, shop and so on, so check the times of trains from west-hampstead station before you go. If you end up with spare time at newcastle airport you can always read or play a game again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of west-hampstead station.
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