Hi there, there is a regular train service from St. Pancras (up to 4 trains an hour) and they take 30 minutes to get to Luton Airport Parkway, from there I believe you have to take a shuttle service to the airport terminal.
Alternatively, and as a cheaper option, you can take a coach (National Express) from the central London Victoria Coach Terminal to the airport terminal. There are 3 services an hour, but the journey takes around 70 minutes.
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