I think some people are confused by a company vehicle that is used for private milage and a company car.
Roxie has it right. A company car is a perk and as such it is yours for the duration, your car fleet manager will advise you but there shouldn't be any problem letting your wife drive. Two points
1) I would look at the tax implications, it may be cheaper to just get a private car. remember apart from tax there are no other overheads.
2) You will be asked what milage you expect to do, make sure that any penalties for going over it will be paid by the company.