They are acting illegally, they are not insured or Licensed to do so. Take the registration Number and report it to the police. If you ask for a taxi in your hotel, and you are offered anything other than a licensed cab, refuse and notify the hotel management. The Licence plate of the cab is fixed on the rear, it gives the cab registration number, Licence number,date that the plate expires and the metropolitan police logo. The cab number is also displayed in the passenger compartment.
Drivers also wear a badge with a number.
If it doesn't have a TAXI sign on top it is not a Taxi.