To transfer your .UK domain name from Internetbs to another registrar requires changing what's called the registrar tag or IPS tag to the new company's tag. It works a bit differently from gTLD transfers, like for a .com domain.
Once you have the IPS tag, please login to your account, click on your domain name, click on ‘Change Tag’ and enter the IPS tag of the new registrar and click on ‘Transfer’ and the domain will be transferred to the new registrar.