The penalty for permitting or allowing an uninsured person to drive your vehicle is harsh, as you have found out,
Your proper course of action would have been to telephone the police and report the vehicle as stolen each time your partner drove it.
This is an example where the vehicle owner believed the friend to be insured:
" I allowed a friend to drive my car believing they were insured when they were not, where do I stand?
You are guilty of the offence of permitting the vehicle to be used by an uninsured driver and face a licence endorsement of 6-8 points and a fine."
You knew your car was being driven by an uninsured, unlicensed driver and failed to report him to the police, or take steps to stop him.
As you failed to report the car as stolen the court has acted quite properly, the fine and ban are correct.
You can appeal but I don't expect it to do you any good.