To transfer any domain name ending with .uk you will need to get in touch with your current provider and request that they change the IPS Tag for the domain name to be:
AFFHOST
Once the tag has been changed to the above you will be able to complete the domain name transfer over to us.
Nominet are the '.uk' domain name registry and oversee all .uk domain names. If you are having any issues releasing your domain to us, we recommend getting in touch with them.