Depends whether it's your car, supplied by a firm, independent,location, what hours you are prepared to work.
I know a couple of taxi drivers, both with their own car working for a small company.
Tips especially at Xmas can be a lot. Also getting vouchers on your fuel means for at least one of them all the family Christmas presents are paid for by the vouchers.
leading up to Christmas and new year you can earn £200 to £300 per night after costs.