As a Consultant you are given £600 flexible benefits to spend. This covers (not fully) the car scheme. The rest of it will be made up out of salary sacrifice. You can expect to sacrifice £100-300 of salary per month, pre-tax.
The cars at your disposal will be similar to cars at the disposal of grades above you, as it is done on a salary sacrifice scheme. All available cars are carbon efficient.
Once you reach Senior Consultant, your benefits will increase to £5000 and therefore should be able to fully cover a car in this (without the need for salary sacrifice).
Hope this helps