Gone are the days of popping into your local DVLA, having a chat with the clerk and skipping out with your MV's reg no. Everything goes through a central database and is checked to see that your vehicle complies with construction and use legislation, particularly size, weight, lighting and braking systems, and that vehicles are legitimately released. All is not lost, as there are exemptions that allow you to legally register your vehicle. You will need to study the numerous variations available in the bodytype, taxation class, and wheelplan categories ;-)
Terry