.CO.UK is country code second-level domain for the UK where .co stands for commercial. The domain extension is intended for websites located or doing business in the United Kingdom. As the .uk domain the registry for the .co.uk extension is Nominet.
Requirements to register a .co.uk domain
Currently there are no requirements for the .co.uk domain. Everyone can register the second-level UK domain. If you would like to register one yourself you can do that from the page here.