What ever you do, do not sign a Voluntary Surrender form as this waives your half rule rights. The Regulator is right.
You can fill out the Voluntary Termination (VT) as this relates to the Half Rule. If I were you, I would firstly get the car checked by the garage to see if any money is owed before submitting the form. Then there will be no ugly surprises.
I would also write to the bank directly to make sure they are well aware that the HP is terminated under the half rule.