This answer will explain what IPS tags are, and how they are used with .uk domain names.
Internet Provider Security tags are used by Nominet
to identify the hosting company linked to a specific .uk domain name.
is the Internet Registry for .uk domain names, and all hosting companies which host .uk domains are required to have an IPS tag (also known as a Nominet Provider tag or IPS key) which shows membership of Nominet. The tag is used to identify the company responsible for management of a .uk domain. Its format is a single alpha-numeric sequence, all in capital letters. For example, the 123-reg IPS tag is 123-REG
Some providers have a "handshake" system where they will need to approve the transfer in and therefore the IPS tag may not update automatically. Please check with your provider if they have this process in place so that they can approve the transfer once you have changed the TAG.