Probably late in the day, but I recently stumbled across the quite brilliant Hailo taxi app co-founded by three cabbies in London. For those not familiar with this app, it allows you to select a black cab in the near vicinity, indicating how many minutes it will take to reach you and gives the taxi details to look out for when it arrives.
After arriving at your destination you can then automatically pay with your credit or debit card as the fare on the meter is charged straight to your account. Without any additional mark-ups and a copy of the receipt emailed directly after the journey I found it to be a joy in its simplicity and effectiveness – especially when using it for the first time on a late, wet and windy night. And of course the cabbies win too as they find more passengers on the street, with the added benefit of avoiding traffic on the way through an alert system.