During the last 6 months I have researched the TfL Executive Car Rental market in some depth.
I have spoken to every Executive Car Rental company that supplies TfL licensed vehicles. Most that advertise Mercedes as being available for hire immediately in trade publications fail to have any Mercedes-Benz E Class available.
Most TfL Executive Car Rental companies will encourage self employed private hire drivers to simply call regularly to try to obtain a car. In my opinion the Mercedes-Benz E Class is the best private hire vehicle available for the executive market so this may explain why demand outstrips supply in London.