Domain name registrations directly under .uk launch tomorrow morning at 0800 London / Dublin time.
Nominet, the .uk registry, have put in place a “grandfathering” system to decide who has the right to register a .uk domain name.
Essentially if you have an existing .co.uk domain name registered before October 28th 2013 you get first dibs on the matching .uk domain. In fact you’ll have 5 full years to make use of your claim on the string.
You can check which domain name qualifies for a particular string using a tool on their site here.
If you qualify for the .uk but don’t want to use it you can transfer it to someone else, but you’ll need to register the .uk domain first.
This and other questions are covered in their FAQ.