Unlike most other registries, Nominet do not allow the legal registrant on a domain to be changed on the fly as seen with other TLD/ccTLDs.
The Nominet online service should be used for this which can be found at
https://secure.nominet.org.uk/ and the email address should be the one used for the registrant contact.
If you are unsure of the email address you can update or check this within your account via the
My XILO portal by clicking on Manage Domain on the domains list for the name you want to check, and then Management Tools and Contact Information.
From that page you can see the current registrant information and email address. You are able to update the email address if incorrect and those changes are immediate.
Once logged in to the Nominet portal, you are able to make the required changes to the registrant name. If the legal entity changes either by name or entity type, Nominet make a charge of £10 + VAT, which you would pay them directly.
If you have any problems when using Nominet's online service, please contact the registrant support number on 01865 332244 or +44 1865 332244. Alternatively, you can email them, their email address is "nominet@nominet.org.uk".
As an accredited registrar, we can also change this for you for a reduced fee of £5 and this can be arranged by contacting Domain Name Support via your portal. Verification may be required for the losing and gaining registrant.
