.UK is the Country-Code Top Level Domain representative for the United Kingdom, intended for companies and individuals associated with it. Some Country-Code Top Level Domains cannot be registered without additional Second Level Domain that divides them into smaller categories. In the case of the .UK domain, you can’t register a domain name directly under the Top Level Domain, but under a Second Level Domain, thus your actual site name will be the Third Level Domain. The registry for .UK domains is Nominet UK and there are more than 10 million .UK domains currently registered, mostly with .CO as SLD, followed by .ORG and .ME. The .UK domain is the 4th most widely used TLD. From all .UK extensions most popular is the .CO.UK domain.
.CO.UK Registration & Usage
As mentioned, domains with the .UK extension cannot be registered at the second level. However, most of the .UK domains can be freely registered by anyone. For the .CO.UK and .ORG. UK registration there are no restrictions, although the .CO.UK domain is indented for general use (mostly for companies) and the .ORG.UK is meant for non-profits. But these requirements are purely formal. The .ME.UK domain is reserved only for individuals and can’t be registered by firms. To register these domains you don’t have to be a UK citizen or a UK-based company. NET.UK is reserved only for ISPs and similar companies, AC.UK is for academic institutions and SCH.UK is for primary or secondary schools. LTD.UK and .PLC.UK are restricted only to UK companies. However, these and other more restricted .UK domains are not very popular for registration.