.CO.UK DOMAINS:

Techincally, a co.uk domain is actually a second-level domain. The central body in charge of the .uk domain decided to subdivide it into categories, such as .gov.uk, ltd.uk etc. The .co.uk was chosen as the main Uk domain by the original govening body, which was made up of part-time volunteers. In the 1990's the domain was regulated by NominetUK, the .uk Network Information Centre, which continues to administer the domain to this day.

However, the .co.uk domain is used as a top-level domain in that it works in the same way as a .com, .net or .org. Originally intended only for use by companies, it is now the fifth most popular top-level domain worldwide with over seven million registrations. Domain users no longer have to have a connection to the UK in order to register a co.uk domain. Also, the top-level domain .uk was released in 2014.

Links:

Wikipedia Entry
Nominet UK
123-Registrar
UKReg