To change the Legal Registrant of a .uk domain name, this must be done through Nominet (the Internet registry for the .uk domain name extension). You can do this via your online. Please note that this involves a charge of £10.00 + VAT.
Log into your Nominet account. If you do not have an account, choose the 'First time logging in or forgotten your password?' option and enter your email address which is associated with the domain and complete the registration.
Once you are logged in, located under 'Your Domains', choose 'Registrant Transfer'. Select the domain name(s) that you wish to change. You will be asked to confirm the new registrant's email address and who will be paying the transfer fee.
When you have completed these details, confirmation will be sent to the new registrant. The new registrant will then have 5 days in which to agree to the transfer. If after this time the transfer has not been agreed, Nominet will issue a refund and you will need to start the process again if you still require the registrant transfer.
If the new registrant accepts the transfer, you will receive an email confirmation when they have completed the process.
Please also refer to the information given on Nominet UK's website - http://www.nic.uk/registrants/maintain/transfer/