There is no automatic entitlement to a company car. This would be between you and your employer, so if you have a company car it should be clearly stated in your written employment contract as an express term. Sometimes, rules about vehicles are found in a separate vehicle policy or staff handbook. If there is no written term referring to your right to a company car, consider whether there might be an implied right based on 'custom and practice'.
During maternity leave, women are entitled to the benefit of all their terms and conditions except for pay. So, for example, if you have a contractual right to a company car, you should get to keep it during your maternity leave.
Your employer must not breach equality laws when deciding who should get a company car.
If a company car is available to full-time workers, it should also be offered to comparable part-time workers, unless the employer can justify treating part-time workers differently on objective grounds.
The provision of a company car will be considered by HM Revenue and Customs as a taxable benefit.