To take benefit of these prices you will need to prove you are not a business traveller, en route to the airport, so you will need to give either yr flight details or yr holiday booking reference. I don't think you will need to give this when you arrive at streatham-common station but you may be asked to give them on yr travels from streatham-common station to the airport.
Going to the airport on the train is great, you can sleep, read (even at night), play a game, or just stare out of the window... The other caveat to buying these inexpensive tickets from streatham-common station to the airport is that there must be at least two grown ups travelling together. But another great benefit is that kids go free. All kids under 5 go free, and then one kidover 5 but under 16 per grown up travelling on the train to the airport.
be sure to leave plenty of time to get to the airport early enough to eat, shop etc, so check the times of trains from streatham-common station before you go. If you have spare time at the airport you can always read or play a game again! As you have bought yr ticket in advance, you don't need to fret about the opening times of streatham-common station.
You can get a tickets this way from streatham-common station to Gatwick, Luton, Stansted, Manchester, Birmingham, Glasgow, and Newcastle. my employer don't sell tickets from streatham-common 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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