.ORG.UK is another country-code second level domain for the UK similar to the .CO.UK. The .org stands for organization which means that it is intended for non-profit organizations. ORG.UK is one of the extensions required by Nominet if you would like to register the same domain with the .UK extension. You can read more about the requirements for the .UK domain here.
Requirements for registering a .ORG.UK domain
As the .CO.UK domain the ORG.UK can also be registered by everyone. However, it is usually used by websites located or doing business in the UK. If you would like more on how to register a domain, you can read our article here.