If when checking the availability of your .uk domain our domain checker is showing it is "Unavailable" when you're certain it is not registered already, this suggests it is a reserved domain.
A reserved .uk domain is one where the matching .co.uk, .org.uk, or .me.uk already exists. The owner of one of these domains are entitled the claim the newly available .uk version, the ability to register it has been reserved for you until July 2019.
For example if you currently own mycompany.co.uk then you may wish to also claim mycompany.uk, this is your exclusive right. After July 2019 anyone will be able to register the .uk version of your domain.
If however you wish to register a totally new .uk domain, for which no equivalent .co.uk exists, they are available to register in the normal way.