To take advantage of the best prices you will need to prove you are a leisure traveller, en route to the 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 provide them on your journey from the station to the 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 restriction to buying these inexpensive 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 child over five but under sixteen per adult travelling by train to the airport.
be sure to leave enough time to get to the airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you end up with time to kill at the airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to fret about the opening hours of the station.
You can get a train tickets this way from streatham-common station to Gatwick, Luton, Stansted, Manchester, Birmingham, Glasgow, and Newcastle. My employer don't sell train tickets from the station to Heathrow and some other airports, because you can't get one train ticket to get you all the way there, boo! Damn you Heathrow Express!
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