To take benefit 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 provide either your 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 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 caveat to buying these cheap train tickets from streatham-common 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 adult travelling on the train to the airport.
be sure to 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 have time to kill at the airport you can always read or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of streatham-common station.
You can get a train tickets this way from streatham-common station to Gatwick, Luton, Stansted, Manchester, Birmingham, Glasgow, and Newcastle. Holiday Extras don't sell 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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