To take advantage of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either your flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at streatham-common station but you may be asked to provide them on yr journey from streatham-common station to the airport.
Going to the airport by train is great, you can take a nap, read (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these cheap tickets from the station to the airport is that there must be at least two grown ups travelling together. But another great benefit is that children go free. All children under five go free, and then one kidover 5 but under sixteen per adult travelling by train to the airport.
make sure you leave enough 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 your train ticket in advance, you don't need to worry about the opening hours of the station.
You can get a train tickets this way from the 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 ticket to get you all the way there, boo! Damn you Heathrow Express!
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