Five things move the price, and only one of them is distance. Pickup location sets the baseline. Vehicle size comes next — a saloon carrying two people with two cases costs less than an eight-seater carrying a family with skis, and choosing a bigger car than you need is the most common way people overpay. Passenger and luggage numbers determine that vehicle choice, which is why we ask for both at booking rather than guessing. Extras such as meet & greet inside the terminal, child seats or additional stops are priced transparently and shown before you confirm. Finally, the direction of travel matters slightly: an arrival into Luton includes flight tracking and free waiting time for passport control and baggage reclaim, which a departure run does not need.
What does not change your fare with a fixed-price booking is traffic, the time of day, or how busy the airport happens to be when you travel. That is the practical difference between a fixed fare and a meter.