To change the registrant or ownership of a .uk domain name, you need to contact the  Registry for .uk domain names (Nominet) and ask them to change the registrant.

All the information and the instructions on how to change the registrant are available on the Transfer page of Nominet’s website.

Once the Registry has approved the change of ownership, you must send Aruba the documents, proving that the change has been completed so the recorded details can be updated, by fax to this number +39 0575/862100 or by opening a Ticket from the Aruba Support section:

If the New Owner wishes, he/she can change the Billing details, the contact email address and the login and password by sending the following documentation duly filled in** and signed   to Aruba to the fax number above.

Change Billing Details 
Change Login and Password 
Change Contact Email Address

If you do not wish to Change the Billing Details or the Login and Password, the new owner must specify this in the request by checking the relevant boxes.

To change the ownership of a MySql or MS Sql database you must follow the instructions in the article: Change ownership of MySql and MS Sql database.
IMPORTANT: Please enter the new email address in the form in BLOCK CAPITAL LETTERS. If the details entered are not clear / correct, the request may be rejected.

So that the documents are accepted, they must be duly filled in and signed by the  new owner.