Is the owner of the car the policyholder or the person whose name is on the log book of the car?

The registered keeper will be the person whose name appear in the log book/registraton certificate. The policyholder is the main driver which in your case is your mum. You are the legal owner of the car as the finance is in your name. However the registered keeper could be either your mum or yourself; chances are that your mum is the registered keeper because many insurers will not quote you if the policyholder is not the registered keeper of the car. To confirm this though, all you have to do is check the log book and the name which is stated on there is the registered keeper. For more information, see the article on registered keeper/policyholder/legal owner to understand the differences.