Nominet UKNominet UK is the .uk domain registry. It is a not-for-profit company limited by guarantee. It has 'members' who act as shareholders. Anyone can become a member, but most members are ISPs who are also tagholders (see below).
Unlike the model in other countries, by and large customers wishing to register a domain do not approach Nominet directly, but register the domain via a 'tagholder'. Loosely, this is an organisation with a credit account who have registered a PGP key with Nominet.