When you are travelling alone, carrying only a cabin bag, starting from somewhere already near St Pancras, and flying at a reasonable hour. In that specific case the train is quick, cheap and entirely sensible, and there is no reason to pretend otherwise. Change any one of those variables — a second traveller, a hold bag, a child, a 5am departure, a late-landing return — and the taxi becomes the straightforward choice rather than the indulgent one. Most Luton journeys involve at least two of them.
If that describes your trip, you can book your Luton Airport taxi with a fixed fare confirmed up front, or read more about our Luton Airport transfers.