If you want to transfer your .uk domain to another registry, you need the TAG from your new provider. If you have this new TAG follow the below steps to change.
1. Login to PowerPanel
2. Find the subscription from your .uk domain
3. Now click on the domain you want to transfer out
Below the details you will find the button "Terminate subscription"
4. Choose the option "Move away subscription"
Now you got a new field where you can enter the TAG from your new provider.
Note: Make sure the domain has been accepted. If not, the domain will be terminated on the expiry date. If the transfer of the domain has failed remove the termination before end date (if you want to keep the domain.
You can use the green button to Withdraw the termination.