What are my options if the finance company illegally repossessed my car?
Conduct that harms other people or their property is generally called a tort. It is a private wrong against a person for which the person may recover damages. The injured party may sue the wrongdoer to recover damages to compensate him for the harm or loss caused. A conversion occurs when personal property is taken by a defendant and kept from its true owner without permission of the owner. Conversion is the civil side of the crime of theft.
You might consider paying (under protest) the fee necessary to get your car back and then sue the Repo Company for conversion in small claims court.