Have a look at the court cases at Reading Magistrates Court.
Several Über London drivers get prosecuted there every month for driving without a licence or insurance, and Über were refused a licence to operate in Reading. They could use their London licence to pick up passengers who want to travel to London. London firms who specialise in the airport market do this, but people generally book their travel to the airport at the same time they book their flight tickets, and Über aren't geared up for that.