Unless there is full reimbursement of fuel provided for the private use of a company car there is a benefit in kind charge based on a fixed figure of £24,600 which is multiplied by the CO2 emissions percentage that is used to calculate the company car benefit for that vehicle. For a high emission car that percentage can be as high as 37%, resulting in a benefit in kind charge of £9,102 and an income tax bill of £3,640.80 for a higher rate taxpayer.

Even with current fuel prices, that would be an awful lot of private mileage, so the employee should consider reimbursing the employer using the HMRC advisory fuel rates https://www.gov.uk/guidance/advisory-fuel-rates by 5 July 2022 for 2021/22.