Why do I have to enter a "Registrant" for .UK domains?
Nominet, the central registry for .UK requires that a "registrant" contact in the UK is set when a domain is registered. This registrant cannot be changed later through your Dynadot account. Instead, you will need to create an account with Nominet using the email address you registered your .UK domain with. From there, you can make changes to the registrant information (there may be a fee).
A valid registrant name requires a minimum of 4 characters.
If you are registering a .UK domain you have rights to, you will need to enter the same registrant name that you used for your .CO.UK, .ORG.UK, or .ME.UK.