To take advantage of the best prices you will need to prove you are a leisure traveller, en route to heathrow airport, so you will need to provide either yr flight details or yr holiday booking reference. I don't think you will need to give this when you arrive at clacton-on-sea station but you may be asked to give them on yr journey from the station to heathrow airport.
Going to the airport by 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 kidover five but under 16 per grown up travelling by train to the airport.
be sure to leave enough time to get to heathrow airport early enough to eat, shop etc, so check the times of trains from clacton-on-sea station before you go. If you have spare time at heathrow airport you can always read a book or play a game again! As you have bought your train ticket in advance, you don't need to worry about the opening hours of clacton-on-sea station.
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