Average number of miles by a company car is 18,400 in 2019 (source: https://www.racfoundation.org/motoring-faqs/mobility#a25)
Average petrol cost in December 2020: 115.1p (source: https://www.theaa.com/driving-advice/driving-costs/fuel-prices).
A Ford Fiesta, the most popular company car in 2020, achieves 52.3 miles per gallon (source: https://www.parkers.co.uk/ford/fiesta/review/mpg-running-costs/).
4.54609 litres in a gallon = 12.18 miles per litre. 18,400 / 12.18 = 1510.67 litres required to do 18,400 miles. 1510.67 * 115.1 = £1,738.78 total cost
Average cost per mile on a Nissan LEAF is 3.9p (source: https://www.goultralow.com/home-charging-tool/). 18,400 miles * 3.9p = £717.6 cost to charge.