Two letter names are not allowed -- they are reserved for ISO country codes.

Two character names are allowed. i.e. 3x.co.uk where the format is not aa.foo.uk.

All second level names of .uk are banned from being used as third level names. i.e. nhs.co.uk would be banned as it is a second level name.

All top level domains are banned from being used as third level names. i.e. net.org.uk and org.co.uk are not allowed.

All one character domain names (ie [a-z0-9].xxx.uk) are reserved for possible future sub-domain usage. The only one currently delegated is X.CO.UK. The owner shall still maintain it unless they volunteer to relinquish it.

There shall be at least two nameservers for the domain (with addresses specified in the request) operating at the time the request is submitted. There is no restriction on the physical location or IP address of these nameservers.