Far from being an expert but I believe the correct rate is 45p per mile for business usage of a personal car. This applies whether its electric or diesel/petrol, which used to be great for electric car owners but as others have said the cost of running an electric car seems similar to diesel/petrol at the moment.
The electricity reimbursement for business usage of a company car is 8p per mile as you have said, but you don't have a company car. Perhaps your employer will only pay 8p per mile as they have increased your salary for you to purchase the car.