You may already be aware that each TLD (Top Level Domain) and ccTLD (country-code Top Level Domain) is governed by a different organization. The organization that governs .uk domains is Nominet. The TAG system was invented by Nominet and for this reason IPS TAGs only apply to .uk domains.
A TAG is composed of one word, all in capitals, that uniquely identifies your registrar (registration agent). When you register a .uk domain the TAG of the registrar is linked with that domain. Only the TAG holder associated with a domain and Nominet can change the registration details for the domain; similarly only the current TAG holder (and Nominet) has the authority to change the TAG itself. If you want to transfer your .uk domain to us, you will have to get your current registrar to change the TAG to WEBHOSTUK