WHOIS Data Validation notification comes from the Nominet registry, which manages .UK domains. They have stringent requirements for the accuracy of contact details for domain registrations, but when they send a validation notification, they haven't been able to validate the registration information saved for a domain using the standard public sources.
To update the information you will need to login to your Nominet account using the link included in their email, or contact them with documentation that the provided address details are accurate. We are not able to contact them for you or inquire about the issue due to privacy concerns.
Should you need to update your registrant contact details at iwantmyname, please log in to your account. Click on the domain to be updated, on its page in the dashboard, click on edit contacts.