When trying to register .co.uk domain with enom module you get errors like:
ProcessRegistrarCommand: Registrar Error eNom: Missing or invalid required parameter: uk_legal_type AND
ProcessRegistrarCommand: Parallels Plesk Billing Module Error eNom: The MB Module ENom failed to return data.
This means you have tried to add package using admin area. You need to use clients order page where you will be offered to enter additional attributes for .co.uk domain registration. You may reach order page by logging as a primary contact and clicking "Go to Store" button on the left side menu panel.