Wikipedia
.uk is the Internet country code top-level domain ( ccTLD) for the United Kingdom of Great Britain and Northern Ireland. , it is the fourth most popular top-level domain worldwide (after .com, .de and .net), with over 10 million registrations.
In October 1984, RFC 920 set out the creation of ccTLDs generally using country codes derived from the corresponding two-letter code in the ISO 3166-1 list. GB is the UK's ISO 3166 country code. However, the .uk domain had been created separately a few months before the compilation of the ISO-derived list. Consequently, .gb was never widely used and it is no longer possible to register under that domain.
New registrations directly under .uk have been accepted by Nominet since 10 June 2014 08:00 BST, however there is a reservation period for existing customers who already have a .co.uk, .org.uk, .me.uk, .net.uk, .ltd.uk or .plc.uk domain to claim the corresponding .uk domain, which runs until 07:59 BST on 10 June 2019.
.uk has used OpenDNSSEC since March 2010.