When changing a .UK domain to use nameservers based on that domain you also need to provide the IP addresses of the nameservers (which need to be different), these form the GLUE records which hold it together.
eg:
dns1.mydomain.co.uk -> 123.45.6.78
dns2.mydomain.co.uk -> 123.45.6.79
If you can provide your nameservers and IP addresses we can action this change for you.
