In case you were NOT aware, since October 2001 it has been ILLEGAL for any company to trade as car services of any sort, unless they have a licence issued by the Public Carriage Office.
More on the P.C.O at http://www.tfl.gov.uk/pco/
Now I cannot stress this Strongly enough, Londons 2 largest companies (and quite a lot of smaller ones) do NOT have licences (at the time of writing) and they are operating ILLEGALLY, and if you use them, you or your clients have no Legal recourse against these companies.
These companies are relying on the fact that you are ignorant of UK law, and they assume you will use them because you dont know any different.
I urge you to check the car service you use, are they Licensed?, and dont just take their word for it, check at the P.C.O website