Voluntary surrender of vehicle

I voluntarily surrendered my vehicle in February 2016 due to arrears on my account. The agreement was terminated on March 2016 and i was told i can not resume possession of the car even if i payed the arrears because there is no longer an agreement in place(it has been terminated). I was never given such notice. The car was only valuated in August 2016 and sold in October 2016 and i was left with a huge short fall amount. Is this how voluntary surrenders work, did bmw follow the appropriate steps according to the NCA?