If the .co.uk or .org.uk version has been registered then the .uk version will be reserved until June 2019.
The owner of the .co.uk version has priority to register the .uk version over the owner of the .org.uk
So if you think you have found a good .uk domain, chances are you will have to wait until June 2019 to register it, unless of course someone beats you to it.
I think nominet will be spammed with millions of automated domain reg requests on that date, so all the good domains will be taken near enough instantly