Booked cars
Private hire eg Uber or Addison Lee can be better. They often (usually) quote a fixed rate, will meet you at the arrivals with a sign and drivers usually keep an eye on the arrivals board to check your progress.
They may not be cheaper, but they can be. They do need to be booked in advance (this is not a bad thing).
Your journey may take longer as the cars are not taxis and cannot use bus lanes, for example.
Avoid taxi/car touts at stations and pre-book if you can.