To take benefit of the best prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to give either yr flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to provide them on your journey from west-hampstead 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 travel scrabble, or just gaze out of the window... The other restriction to buying these cheap tickets from west-hampstead station to glasgow-international airport is that there must be at least two grown ups 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 16 per grown up travelling by train to the airport.
make sure you leave enough time to get to glasgow-international 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 glasgow-international airport you can always read a book or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening times of the station.
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