From my understanding there are two specific suppliers I am aware of which are Avis and National which can be booked via a Broker such as Auto Europe or by going direct whom will accept either a cash deposit which is likely to be around £400 or will accept your mothers credit card as a deposit providing she is present at the time of picking up the car. What I would recommend in this instance is before making the booking on line call one of the organisations mentioned above to confirm that they still accept this. I would also recommend you ask them to e-mail or fax you confirmation that they are prepared to accept your mother’s credit card although she will not be the driver to save any issues locally when you pick up the car.